June 16, 2007
Formation, Dynamics, and Characterization of Nanostructures By Ion Beam Irradiation
By Dhara, S
Ion beam irradiation is a potential tool for phase formation and material modification as a non-equilibrium technique. Localized rise in temperature and ultra fast (~10^sup -12^ s) dissipations of impinging energy make it an attractive tool for metastable phase formation. As a matter of fact, a major component of materials science is dominated by ion beam methods, either for synthesis of materials or for its characterization. The synthesis of nanostructures, and their modification by ion beam technique will be discussed in this review article. Formation of nanostructures using ion beam technique will be discussed first. Depending on species (e.g., mass and charge state) and energy range, there are various modes for an energetic ion to dissipate its energy. The role of the electron will also be covered in this article as a basic principle of its interaction with matter, which is same as for an ion. By using a simple reactive ion beam or electron induced deposition, a secondary phase can be nucleated by ion beam mixing techniques, either by using inert gas irradiation or reactive gas implantation on any desired substrate. Nucleation of secondary phase can also be executed by electron irradiation and direct implantation of either negative or positive ions. Post implantation annealing processes are required for the complete growth of clusters formed in most of these ion irradiation techniques. Implantation processes being inherently a non-equilibrium technique, defects always have a role to play in phase formation, amorphization, and beyond (blister formation). When implanted with large energy, even electrons, one of the lightest charged particles, also manifest these properties. Electronic and nuclear energy losses of the impinging charged particle play a crucial role in material modification. Doping a nanocluster, however, is still a controversial topic. Some light will be shed on this topic with a discussion of focused ion beam. Keywords focused ion beam, dynamic annealing, phase transition, doping, amorphization, blister1. INTRODUCTION
New physics is realized in nanoclusters with respect to structural (new crystal structure),1 optical (quantum confinement effect with cluster size less than the exciton Bohr radius,2,3 surface plasmon resonance in nanosized clusters),4 electronic (discrete density of states), electrical (single electron transistor),5 thermal (melting point decreasing with decreasing cluster size,6 specific heat anomaly in small clusters leading to experimental realization of surface acoustic phonons),7 mechanical (crossover in hardness with decreasing cluster size at nanoscale regime showing inverse Hall-Petch effect)8 and magnetic properties (few atom clusters of diamagnetic element showing ferromagnetism).9 In other words, materials science has observed a renaissance with the advent of nanoscience and perhaps achieved a real identity in the world of science. These are all about physical properties, which are, of course, an important aspect of materials. However, growth of nanostructures received immense importance in order to realize unprecedented physical properties in materials.
There are two ways the science has learned to grow nanostructures. One way is by reducing dimension, namely, from bulk to thin film and then reducing its thickness along with other in- plane dimensions. Drive in this direction started in the late 1950s and is known as "top down" technology in the language of present days.10 The other way of forming nanostructures in a consolidated way started a bit late with "bottom up" technology. In this process, instability in phase formation nucleates the nanosctructure in most of the cases. Thus, the basic principle of this technology is built on non-equilibrium process where multiple phases can form. Ion beam interaction is a non-equilibrium technique as thermodynamic parameters, namely, temperature during ion matter interaction, pressure during phase formation or nucleation at the depth of the bulk fluctuate in this process. Thus, ion beam technique is a natural choice for the formation of nanostructures. The typical time scale and lengths over which a single cascade (displacement sequence of collision events) of ion matter interaction occurs are ~10^sup - 13^-10^sup -11^ s and 10^sup -9^-10^sup -8^ m. Typical increase in temperature in a small volume of nano-metric length scale are ~10^sup 4^-10^sup 3^ K.11 Thus, fast mean relaxation process (~10^sup -12^ s in a single cascade) gives a typical quenching rate of ~10^sup 15^ K sec^sup -1^. Such a fast quenching rate in a thermal spike is probably the highest among currently available techniques for producing metastable phases. Meanwhile, such a short time of relaxation restricts the kinetic condition for the phase formation so that only amorphous or structurally simple crystalline phases are obtained.12 Both the time scale and lengths favor the formation of nanostructures in the ion irradiation process. Formation of defects and their evolution is also inherent to this process and needs addressing.
Another major advantage of ion beam synthesis technique is the area and depth selectivity of phase formation in a substrate. Area selectivity can be achieved either by focusing an ion beam or by using a suitable mask. Ion energy may be tuned to grow nanostructures at different depths. The easiest way to look at the nucleation of nanophase is with reconstruction of chemical bonds in the cascade (size ~10^sup -9^ m) of mostly organic and inorganic substances during interaction with either ions or electrons. These nanophases can be formed either during growth on substrate, namely, low energy ion beam induced deposition (IBID) or by the impinging electron in the deposition process. Another way of nucleating nanophase is by the supersaturation of atomic species in the "far- from-equilibrium" state of solid (either negative or positive) ion in the immiscible system. It can also be formed by ion beam mixing (IBM) of immiscible systems in the recoil implantation processes leading to self-organization when annealed (relaxed configuration).13 The technical issues of dispersion of cluster sizes will be discussed in light of implantation parameters like variation of temperature, ion flux, and energy during ion implantation, which can control the size of nanoclusters as well as size distribution.14-16 The vast literature dealing with the growth of nanoclusters constitutes section 2.
Defects are generated in non-equilibrium conditions, with ion bombardment functioning as indispensable processing steps either for growth of nanostructure or their device applications. Manifestations of defects in nanostructures are unique due to their energetics for microstructural evolutions and their ability to help phase formations. Electron irradiation has an advantage over heavy ion implantation as minimum defects are introduced in the earlier process. Electrons, being light in mass, penetrate deeply into target materials and dissipate their energy through electronic excitation of the target atom. In-situ transmission electron microscopy (TEM) during electron irradiation is one of the best techniques to address many critical phenomena, dynamically, in solid- state physics. However, in most cases, additional thermal energy, redundant in the case of ion-matter interaction, is a prerequisite for phase formation in the electron irradiation process.
Diffusion limited aggregation (DLA) is also discussed for microstructural evolution and phase stability in the implantation process. Radiation induced segregation (RIS) of atomic species in the non-equilibrium process induces defect migration, which in turn takes the system away from equilibrium, leading to amorphization.12 Interestingly, material beyond amorphization also attracts attention. Formation of voids and bubbles are observed as a consequence of supersaturating defects (displacement damage) and accumulation of implanted inert gases. Incidentally, disintegration and simultaneous accumulation of gaseous components of target material beyond the fluence of amorphization is also reported to form bubbles. Formation of voids and bubbles during ion beam interaction with materials addresses many important issues. In fact, formation of a three-dimensional (3-D) void-lattice has demonstrated the earliest example of self-organization17 in material processing. This is the key concept of the "bottom-up" technology used for modern ultra-small scale device formation. On the other hand, an understanding of void or bubble formation addresses issues of swelling and embrittlement in detecting material failure used in modern technology (e.g., materials used for the nuclear reactor). section 3 deals with the formation of various defects and their evolution in nanostructures by ion implantation.
Final discussion will be on issue of doping, which is not always favorable for nanoclusters because large amounts of energy are required to put a foreign atom in a crystal. Impurities that increase the number of electrons or hole carriers are essential in most bulk semiconductors. Introducing such foreign atoms in semiconductor nanocrystals is tricky, at least via chemical routes. Energetic ion implantation can provide such energy, particularly with mono-energetic (energy spread
2. NANO-PHASE FORMATION BY ION BEAM TECHNIQUES
Under this subsection, nanoclusters grown on substrate will be examined. No attempt will be made to cover formation of nanostructured patterns either using sputtering of material surface by ion beam19 or reactive gaseous ion etching.20 In the 'top down' technique itself, one has control over the kinetic energy of adatoms (atoms being accommodated on the substrate from target or source), or one can make the surface condition such that nucleation is restricted to the island formation stage. With controlled deposition parameters, namely, operating pressure, ambient gas, substrate temperature, and substrate orientation, one can grow nanostructures. Sputtering technique is one of the earliest examples. At low energies momentum transfer by rf/dc sputtering using gaseous plasma or direct impingement of energetic ions (ion beam sputtering, IBS) ejects target molecule, to be deposited on the substrate, in a non- equilibrium process. Only the IBS will be referred in order to concentrate on the ion beam related area. Surface energy of the deposit has to be higher than the combined surface energies of the substrate and at the interface to achieve a good control over the size of nanoclusters. Au,21,22 Fe,23 Ni,23 and Co (Ref. 24) nanoislands, used as catalyst for the growth of other nanostructures particularly one dimensional (1-D) structures,25 are coated by these techniques routinely on varieties of substrates. Ion beam sputtered self-assembled silver nanocrystals on Si nanotips (Figure 1),26 and hexagonal wurtzite (h-) A1N nanorods27 exhibit surface enhanced Raman scattering (SERS). Nanoclusters are also produced in this technique where immiscible system is co-sputtered to form a reasonable dispersion of the grown nanophase. The most important example is the growth of Si nanocluster in SiO^sub x^ matrix.28 Photoluminescence (PL) is recorded in the visible region for these quantum confined Si nanoclusters. Focused ion beam sputtered deposition of Co^sub 71^Cr^sub 17^Pt^sub 12^ and Ni^sub 80^Fe^sub 20^ on tips for magnetic force microscopy are reported.29 We must recognize that the sputtering techniques, being a non-equilibrium energetic process of growing nanoclusters, act as a bridge of the existing knowledge to meet the advanced requirements. In an interesting report, nanocrystalline carbon nitride (beta-C^sub 3^N^sub 4^) is synthesized at low temperature (
Another area will be touched upon for cluster ion deposition with accelerated clusters dissipating the kinetic energy onto a buffer layer, which acts as a target to be evaporated and leads to a soft landing of the cluster on the substrate.32 The advantage of this method is that it is possible to select the mass of the ionized clusters before deposition. Low energy cluster beam is also reported for deposition of clusters with few adatoms with kinetic energy ~meV/ atom.33 In a similar context, clusters are deposited in the form of cermets produced by combining a cluster beam with an atomic beam of the encapsulating material.34 These cluster beams are generated by different techniques, namely, multiple expansion cluster source (MECS)33,35 and gas aggregation.33 To produce clusters of refractory materials by a different evaporation technique, namely, vaporization by the impact of a laser and thermalization of high pressure, He gas is needed.33 Detailed experimental results and simulated structures are described in the review article by Jensen.36
FIG. 1. Scanning electron micrographs (SEM) showing (a) cross- sectional view of the as-grown Si nanotips (inset shows the top view of the as-grown nanotips); cross-section images of Si nanotips covered with silver sputtered for (b) 1 min, (c) 3 min, (d) 5 min, (e) 10 min (inset showing a single tip with the silver nanoparticles), and (f) 20 min (inset showing a magnified image of the Si tip covered in a silver film) in IBS.(Ref. 26 (c) 2005, with permission from American Chemical Society).
2.1 Ion Beam Induced Deposition (IBID)
Growth of nanoclusters on substrate by the modification of chemical bonds of organic or inorganic substances will be discussed in this section. Electronic [(dE/dX)^sub e^] and nuclear [(dE/ dX)^sub n^] energy losses in the multiple cascades are predominant in the case of ion-matter interaction. Changes in the electronic and the structural configurations are initiated by (dE/dX)^sub e^ and (dE/dX)^sub n^, respectively. Typical projectile ranges and energy loss of both light (N+) and heavy (Ga+) ions in a compound (GaN) are shown (Figure 2) using SRIM code calculation.37 Light ions penetrate deeper into the target with respect to heavy ions. Irrespective of ionic masses, nuclear energy loss dominates in the lower energy range, and electronic energy loss prevails at higher energy.
The cross-over occurs at very low energy for light ions (Figure 2). A low energy Ga+ beam with organic compounds or alloy is predominantly a nuclear energy loss process where metal atoms get separated from weak covalent bonds in a major molecular reorganization and get deposited on substrate. FIB induced deposition of electrical contacts to nanostructures for electrical measurements and catalyst growth for further formation of nanostructures using metal containing an organic precursor gas or alloy are reported for Pt (e.g., trimethyl-cyclopenta- dienylplatinum [(CH^sub 3^)^sub 3^CH^sub 3^C^sub 5^H^sub 4^Pt]),38- 40 Au [Au-Si alloy (Au 70 at.%)]41 or W [e.g., W(CO)^sub 6^)].42 These organometallic compounds are solid at room temperature with a low vapor pressure of few Pa. Typically, these vaporized precursors are injected from a gas nozzle, and simultaneously FIB is scanned in the deposited area. A reactant gas may also be used for chemical reaction during the deposition, as discussed later in this section. A schematic of the deposition process is shown in Figure 3. In the case of organic molecules, other constituents can either be released as gases (e.g., CH^sub 4^ and its derivatives) or can segregate (C being the major constituent).38 Low energy Ga+ ions at an optimum current have been used to reduce resistivity and content of the C impurity in the deposits for organic sources. Growth of Pt contact on GaN nanowires (NWs) for electrical characterization and use of Pt as catalyst for subsequent site selective growth of GaN NWs using FIB are also shown in Figure 4.40 In the presence of a reactive gas, FIB induced chemical vapor deposition (IBICVD) of gallane quinuclidine (GaH^sub 3^ : NC^sub 7^ H^sub 14^) as the Ga metal source precursor has demonstrated formation of GaN nanostructures.43 The reader is referred to a recent review article by Tseng44 describing formation of nanostructures by FIB.
FIG. 2. Typical plots for range and energy losses of N+ (light) and Ga+ (heavy) ions in GaN target. Electronic [(dE/dX)^sub e^] and nuclear [(dE/dX)^sub n^] energy losses are shown for both ions represented by respective scattered (open circle and filled square) symbols. Inset shows, electronic energy loss crossing over nuclear energy loss at very low energy for light ion. (Calculated using Ref. 37).
Following a similar mechanism, nanostructures are also reported for gaseous ion implantation in pre-coated substrates. For example, graphite like clusters (GLC) are grown by ~100 keV N+ implantation in polymeric materials.45 A fractal pattern originating from DLA process is shown (Figure 5) in the 2-D polymeric matrix. This structure will be discussed again in the framework of DLA and molecular reorganization for the growth of GLC phase in section 3.2.1. Al nanoaggregates are also formed by our group using He^sup +^ and Ar^sup +^ implantation in oxygen bonded metal-organic complexes (chelates) of aluminum-trisacetyl-acetonate [(CH^sub 3^CO)^sub 2^ CH)^sub 3^Al:Al(acac)^sub 3^] (Figure 6). The majority of C and O are released as CO in the "decarbonylation" process, and metal nanoclusters are nucleated in the process.46 Although O is bonded to both C and Al in the precursor, oxidation to the former is preferred during the irradiation process. This may be due to the fact that the localized temperature during the irradiation process can be raised to the order of ~10^sup 4^ K in the thermal spikes along the ion tracks,11 and at such high temperatures (>/=2300 K), the free energy of formation of CO is less than that for the formation of Al-O bond.47 Ag nanoclusters are formed by He^sup +^ irradiation on organometallic precursor of Ag, Ag(1,1,1,5,5,5- hexafluoroacetylacetonate) tetraglyme [tetraglyme = (CH^sub 3^O(CH^sub 2^CH^sub 2^O)^sub 4^CH^sub 3^)].48 Thin spin-coated films of palladium acetate ([Pd(O^sub 2^CCH^sub 3^)^sub 2^]^sub 3^) are used with Ga+ beams to achieve Pd nanostructures.49 Noble metal nanoclusters have also been reported to form in He^sup +^ irradiated metal containing polymer.50 High energy (~MeV) Au+ or He^sup +^ irradiation of gel films prepared from mixtures of triethoxysilane of Fe and Ni nitrates enacts a precipitation of Fe, Ni particles in a glassy matrix, as these gels contain hydride groups able to reduce Fe^sup 3+^ and Ni^sup 2+^ ions.51 FIG. 3. Schematic of FIB-CVD technique involving precursor and reactive gases with assistance of FIB (drawn after Ref. 43).
Another unique report includes the formation of nanocrystalline diamond in the amorphous (a-) C matrix using low energy (~150 eV) Ar+H^sub 2^+CH^sub 4^ ions above a critical fluence of ~10^sup 19^ cm^sup -2^. Ion beam induced stress and energy fluctuation is made responsible for crystalline nanodiamond formation.52 The presence of atomic H in the irradiation process may also play an important role in nucleation process. Assisted by electron irradiation, nanotiles of SiC are also reported with ~ 100 eV deposition of SiCH^sup +^^sub 3^ on Si substrate at a modest temperature of 875K.53
FIG. 4. (a) SEM image of two-probe GaN NW device with contacts patterned by FIB-Pt deposition, (b) Ion-beam image (Zcontrast) revealing Pt spreading around the FIB-Pt leads (white). Inset shows details of FIB Pt-contact: The nanowire is 63 nm wide, and the width of the contact line is 250 nm. (c) FIB patterned template before GaN growth. From left to right, FIB milled holes, Pt dots and lines. Depth of hole is 150 nm, and thickness of dots and lines are 250 nm. Separation between features is 5 iec. (d) The template in (c) after nanowire growth. Nanowires preferentially grew at the patterned Pt dots and lines, while milled holes did not nucleate NW growth, (e) Individual GaN nanowires grown at FIB-Pt dots. Diameters of dots reduced to 250 nm with same 5 [mu]m spacing. Inset shows a magnified image of spherical particle on the tip of grown nanowire. The diameters of nanowire and the particle are ~60 nm and ~65 nm, respectively (Ref. 40 (c) 2005, with permission from American Institute of Physics).
2.2 Electron Irradiation
One of the most significant methods to ligand stabilize cluster formations of different noble metals (rf-elements) is reported by Schmid,54 wherein metal-organic compounds of Au, Ru, Rh, Pd and Pt (e.g. (C^sub 6^H^sub 5^)3PAuCl etc.) are reduced using B^sub 2^H^sub 6^ or NaBH^sub 4^. As electron-matter interaction is predominantly an electronic loss phenomenon, exposure to an electron beam is used to disrupt the weak ligand bonds in order to prepare small clusters with as few as 13, 55, 147, 309, and 561 atom clusters. These clusters show unique crystal structures (icosahedral, cuboctahedral, decahedral).55 Au55 clusters are prepared by exposing to electron beam, and coherent tunneling is observed in these cluster-assembled systems (Figure 7).56 Ru nanoclusters are grown by electron beam irradiation in spin-cast ruthenium carbonyl polymer ([Ru^sub 6^C(CO)^sub 15^Ph^sub 2^CCPPh^sub 2^]n).57 Pd nanostructure is also formed by electron beam exposure of palladium acetate.58 Ag nanoclusters are formed by electron beam irradiation on organometallic precursor of Ag.48 Preparation of nanorods of cubic iron suicide (FeSi^sub 1+x^ or gamma-FeSi2) is observed on crystalline (c-) Si substrates using an iron pentacarbonyl [Fe(CO)^sub 5^] gas source assisted by electron beam induced deposition at an elevated temperature in an ultrahigh-vacuum (UHV) TEM.59 In an interesting development, the problem of C deposits leading to high electrical resistivity in metal coating using ion beam induced organometallic precursors is partially solved by exposing the inorganic PF^sub 3^AuCl precursor to a focused electron beam.60
As discussed earlier, self-assembled SiC nanotiles are fabricated using organometallic ion beam deposition with simultaneous electron beam irradiation.53 The SiC nanotiles formed on Si(100) are heteroepitaxial zinc-blende SiC (3C-SiC). The electron beam assisted irradiation during the SiC formation has effects on shape, size and density of the self-assembled SiC nanotiles. Instead of using the terminology electron beam induced CVD (EBICVD), used loosely by some authors, the role of desorption energy of electron irradiation will be discussed for the growth of nanostructures.
FIG. 5. Bright field TEM of 100 keV N+ implanted PPO film at a fluence of (a) 1 x 10^sup 16^ cm^sub -2^ and (b) 5 x 10^sup 16^ cm^sub -2^. The crystalline microdiffraction pattern of the clustered region is shown in the outset of (a). Zone axis of the selected area electron diffraction (SAED) pattern is calculated to be . Outset of (b) shows log-log plot for covered area (A) vs. radius (R) of cluster in determining the fractal dimension of 1.30 +- 0.075 for the sample grown at a fluence of 5 x 10^sup 16^ cm^sub - 2^ (Ref. 44 (c) 7999, with permission from American Physical Society).
Electron-irradiation damage in 6H-SiC is reported to form direct Si-Si bonding after prolonged irradiation, where the sample still maintained the well-defined 6H structure. No appreciable indication of direct C-C bonding is observed. These results suggest that C atoms predominantly displace away from Si atoms in the Si-C bond, and the residual Si atoms form small clusters.61 Growth of Si nanowires of ~10 nm width is observed by selective thermal desorption of SiO^sub 2^ on Si (111) substrate, induced by focused electron beam irradiation through a window of 10 nm.62 Electron beam annealing and subsequent growth of self-assembled Si nanostructure is reported for c-Si exposed to high fluence ion implantation.63 The nanofabrication procedure involves annealing of untreated Si(100) substrates at high temperature and short durations using a raster scanned 20 keV electron beam. Nanostructuring occurs because of kinetic amplification of the surface disorder induced by thermal decomposition of the native oxide. Radiation induced disorder of the Si substrate prior to annealing by ion implantation modifies the potential energy surface and thus the growth of self-assembled nanostructures. Lithography free fabrication of Si nanostructures on both n-type and p-type Si(100) substrates is also reported of c-Si substrates containing the native oxide, using electron beam rapid thermal annealing at high temperature range.64 The initial stage of nanostructure growth involves thermal decomposition of the native oxide leading to atomic scale disorder of the Si surface. Diffusive Si species migrate across the surface with the completion of oxide desorption. This is in response to diffusion barriers established on the strained potential-energy surface, nucleating islands at kinetically favored sites. The resuiting square shaped nanostructures, distributed across the entire substrate, are aligned to the (110) direction. With continued annealing the island number and size evolves according to crystal ripening processes. Growth of oriented SnOa nanoparticles on SnU2 nanobelts, used as single crystalline substrates, is reported through electron irradiation in transmission electron microscopy.65 High resolution TEM (HRTEM) analyses indicate that grain growth occurs in the solid state by a two-step process consisting of grain rotation followed by coalescence. The grain rotation is proposed to be induced by the thermal energy provided by the electron beam. Coalescence or oriented attachment occurs preferentially on particles with similar crystallographic orientations of the single crystalline substrate.
FIG. 6. (a) Bright field TEM image of 70 keV He+irradiated Al(acac)3 film up to a fluence of 5 x 10^sup 15^ cm^sub -2^, showing scattered and aggregated clusters along with the nucleation of branches. The inset shows the SAED pattern of the clustered region. The zone axis of the spotted diffraction pattern for Al is calculated to be along [04-4]. Bright field TEM image of (b) 70 keV He+ and c) 130 keV Ar+ irradiated Al(acac)3 film up to a fluence of 1 x 10^sup 17^ cm^sub -2^, showing a network-like structure containing inhomogeneous aggregated clusters of Al (Ref. 46 (c) 2007, with permission from Institute of Physics Publishing Ltd.).
FIG. 7. Optical micrograph of typical device for conductivity in Au55 clusters (Ref. 56 (c) 7997, with permission from American Institute of Physics).
Electron irradiation in Ag+ exchanged glass is also observed to produce colloid of Ag nanoclusters.66 A Ag surface depletion accompanied by an accumulation of Ag, up to a depth corresponding to maximum primary electron range, is reported. Such in-depth migration is due to the formation of an electric field. The electric field assisted diffusion is characterized by a radiation enhanced diffusion (RED) coefficient, which is some order of magnitude greater than the thermal diffusion coefficients.
2.3 Ion Beam Mixing (IBM)
In ion beam mixing, layers consisting of immiscible elements are irradiated with high energy heavy ions of several MeV or energetic inert gas ions of several keV, which induce a series of atomic collisions resulting in collision cascade. This collision cascade induces atomic mixing between immiscible elements and drives the resultant mixture to a state far from equilibrium.
In ion beam mixing there are three important processes involved, namely, ballistic mixing process, thermal spike process, and RED process.67 The ballistic mixing comprises of recoil implantation and cascade mixing. The recoil implantation is an ion-atom knock on event. Here atoms are transported preferably in the beam direction. Cascade mixing is a random (spatially isotropic) process resulting from the movement of high order recoil atoms in the target. The role of cascades in the formation of nanoclusters has already been discussed in the previous two subsections. Here, the role of recoil implantation will be elaborated on the formation of clusters embedded in matrix, granular materials, compound formation by sequential metal and reactive gaseous ion implantation.
2.3.1 Recoil Implantation
Formatsters in the immiscible system and studying their physical properties has the following impetus: dielectrics containing metal nanoclusters are promising materials for optoelectronic devices for their nonlinear optical (NLO) applications. Field enhancement effects associated with the surface plasmon resonance (SPR) in the visible region reported in embedded metal nanoclusters is important for the femto-second response in NLO devices.68 Granular materials are considered to be the candidates for many technological applications. One of the major applications is in giant magneto- resistance (GMR), with small grains of ferromagnetic metals embedded in a nonmagnetic matrix, such that the grain sizes and inter-grain distances are smaller than the electron mean-free path and the spinflip length.69 The first experimental report indicates that metallic Ag clusters within a dielectric matrix by an ion beam mixing process is formed in Ag/SiO^sub 2^ system using high energy (~MeV) Au ions.70 The technique employs irradiation of multilayers and mixing the elements with energetic heavy ions. In this process, impinging ions slow down mostly by nuclear energy loss due to collision with the atoms of the target. Optical absorption measurements demonstrate the capability of the technique to form nanometer-sized metallic clusters at relatively low ion fluences (e.g., below 1 x 10^sup 16^ cm^sub -2^) as compared to a direct ion implantation technique, discussed later. The approach, used in this technique, eventually has triggered many publications where different metal clusters are reported to form in the immiscible dielectric matrix. The formation of metallic nanoclusters is observed in various metal/SiO^sub 2^ samples (Cu, Au, Pt) irradiated with energetic heavy ions.72 Interestingly, the nucleation of Au nanoclusters in silica is also recorded by high energy Si+ implantation in co-deposited Au/silica film.72 Layers of noble metals embedded in various oxide matrices (SiO^sub 2^, Al^sub 2^O^sub 3^, TiO^sub 2^, ZrO^sub 2^) are irradiated with incremented fluences of high energy heavy ions.73 75 The mixing of noble metals with various oxides under MeV Au ion irradiation and the concurrent process of metal segregation are studied by means of Rutherford backscattering spectrometry (RBS) and TEM analyses (Figure 8). RBS analyses show that recoil implantation is the predominant mechanism to inject metal atoms into the oxide matrix. The spreading is controlled either by the recoil implantation or by RED of metal atoms according to their mobility.
In a novel approach, Pt nanoclusters are burrowed in SiO^sub 2^ using inert gaseous ion beam mixing of Pt layer coated on silica substrate.76 We have used a similar technique to grow metal nanoclusters (Figure 9) by medium energy (~100 keV) gaseous ion irradiation in silica matrix having a single metal layer deposited on the silica substrates.77,78 The ion energy is chosen such that the mean range falls in the interface region for the subsurface growth of nanoclusters. Recoil implanted metal atoms are supersaturated in the silica matrix owing to their low solubility. Due to the concentration fluctuations, nucleation of Au atoms occurs and these nuclei can grow directly from the supersaturation. Agglomeration to bigger clusters is observed with the increasing ion fluence. Growth of the cluster size with increasing annealing temperature is also reported with the fact that nuclei, those have reached a critical size grow at the expense of smaller nuclei. This process is known as the coarsening or ripening stage. Schematic of nucleation and further growth is shown in Figure 10.
In an interesting technique, as already described in previous subsection for electron irradiation, Ag and Cu nanoclusters are also observed to grow by gaseous ion implantation in Ag66,79-83 and Cu81,84 ion-exchanged glass. In a similar process, c-Ge nanoclusters are reported to form by gaseous ion implantation in GeO2-9SiO^sub 2^85 glass. Irradiation of doped glass leads to precipitate growth and in this case, ionization energy loss processes are deemed responsible for the growth of the particles (Figure 11). Complex optical properties of Cu/Ag metal nanoclusters are reported86 in a soda-lime glass matrix by sequential Cu and Ag ion exchange followed by He+ irradiation.
Ion beam mixing of Co/Cu multilayers is observed with 1 MeV Si+. The correlation between GMR and anti-ferromagnetic (AF) coupling, as well as the role of enhanced electron scattering at interfaces during these processes are discussed.87 Low energy (~150 keV) Cu+ irradiation in Co/Cu multilayer films leads to the formation of a granular system with broad size distribution of Co clusters in Cu matrix, as evident from the magnetic measurements.881 MeV Si+ ion irradiation on Co/Ag multilayer at low ion fluences induces limited demixing of the elements. On the other hand, at high fluences atomic mixing takes place due to high energy recoils resulting formation of granular structure.89
Self-organized Cu-Ag nanocomposites are synthesized at intermediate temperatures by Kr+ ion beam mixing.90 Ion beam irradiation of energetic inert gaseous ions on immiscible system is studied in Ag/Fe layers91,92 (Figure 12) at room temperature and low temperatures for understanding the mixing mechanism. The low mixing, leading to granular system, is attributed to the demixing and the phase separation occurring in the thermal spike phase of the collision cascade. It implies that the ballistically intermixed Ag/ Fe (also reported for Cu/W)93 interface rearranges due to the subsequent spike induced demixing. Importance of thermal spikes in mixing is proved and average size of thermal spikes of ~5 nm is estimated.94 Cu/Nb, Cu/Bi and Cu/Mo systems are reported miscible in the liquid phase. However, the same is not miscible in the solid state even at 6 K inhibiting demixing process.95 Granular system is also observed in Fe/Ge bilayer grown on SiU2 and bombarded with 150 keV Kr+ ions.96
FIG. 8. TEM image of a SiO^sub 2^/Ag [8 nm]/SiO^sub 2^ film ion beam mixed with 1.6 x 10^sup 16^ Au ions (plane section) and histogram of cluster sizes deduced from measurements on 20 images (Ref. 74 (c) 2002, with permission from Springer-Verlag).
2.3.2 Reactive Ion Beam Mixing by Sequential Implantation
In this subsection, attention will be paid to alloy and compound semiconductor nanoclusters grown by ion beam mixing with sequential implantation. Nanostructures obtained by sequential ion implantation on dielectric substrates without subsequent annealing are Au-Cu,97 Au-Ag,98 Cu-Ni,99 NiCo,100,101 Co-Cu,100 Fe-Co,102 Fe-Pt,103 and Ti- Sn.104 Similar systems are also realized with post irradiation annealing for Co-Ni, and Au or Pd based Ag, and Cu alloys (Figure 13).105 Eventually, alloy nanoclusters are also reported in silica matrix.106
Compound semiconductors of h-GaN nanocrystals are synthesized by sequential implantation of Ga and N ions into dielectric substrates,105,107,108 followed by annealing of the samples in a reactant gas at high temperature (Figure 14). Formation of InN nanoclusters is also addressed in this technique.105 A combinatorial materials synthesis approach is applied to ion beam synthesis of CdSe nanodots by sequential implantation of the elements in SiO^sub 2^. It opens the possibility of studying the synthesis of novel complex embedded semiconductor heterogeneous nanostructures.109 Other nanocrystalline phases are also reported by sequential implantation of the constituent elements-these include InP,"0 ZnS,111,112 CdS,112-114 PbS,112 GaAs,115-117 CdSe,113,1180 -120 and InAs,121 as well as oxides such as VO^sub 2^,122 V^sub 2^O^sub 3^,123 and multi-component ZnAl^sub 2^O^sub 4^.124 Binary metallic alloy nanocrystals have also been synthesized.125,126 ZnO nanoclusters are reported by sequential implantation in various substrates.127 Compound nanoclusters have also been obtained by co- or sequential implantation of various atom types, for example, II- VI, III-V or IV-FV semiconductor in SiO^sub 2^, Al^sub 2^O^sub 3^ (Ref. 128) and Si (Ref. 116) matrices. In an interesting report, sequentially implanted compound nanoclusters of ZnS are reported to nucleate thermally by IBM using high energy ions and even by electron irradiation.129 Point defects formed in the electronic energy loss process of the irradiation techniques help in the nucleation process of the nanoclusters. Coherently oriented nanocrystals of ZnS are grown in ion beam amorphized Si matrix.
Core-shell nanostructure is also another interesting area primarily thought for suppressing surface states in small semiconductor nanoclusters at core with a shell overlayer. In metallic system Cu coated Ag,130 and Au coated Cu,131 nanocrystals are reported in silica by sequential ion implantation. ZnO coated Zn nanoclusters formation is also observed in silica matrix with Zn+ and F+ implantation in sequence.132 Oxygen molecules, produced by F+ implantation in silica, partially oxidize the already formed Zn nanoclusters and form ZnO nanoshells. Ag-S core-shell structure is formed by sequential implantation (Figure 15).105 Monte Carlo simulation has been performed for the growth of core-shell structure by ion implantation technique.133 Because ion implantation is inevitably accompanied by a specific level of collisional mixing, there is a competition between a coating of the pre-implanted nanoclusters and a random arrangement of elements for sequential implantation of two ions (e.g., X and Y impurities). As envisaged core/shell formation is favored if (i) the solubility of Y exceeds the solubility of X in the matrix, (ii) the atomic mass of Y is small, that is, low collisional mixing, and (iii) for rather low ion flux and/or high second implantation temperature, which favors a compensation of displacements by thermodynamically driven atomic movements. CdS and ZnS nanoclusters, in a unique structure wollow core, are reported by sequential implantation in SiO2 at room temperature and subsequent thermal annealing (Figure 16).129 A possible explanation for the occurrence of hollow particles involves sulfur adsorption at defects in the crystalline lattice of the pre- existing metal (or semiconductor) colloids. Sulfur, at sufficiently high vapor pressure in the implantation condition, is adsorbed onto nucleated vacancies created during irradiation. Other observations eliminate radiation damage alone as the origin of the voids. FIG. 9. Transmission electron micrographs of Au clusters grown at a fluence of 5 x 10^sup 16^ cm^sup -2^ and annealed at (a) 973 K and (b) 1173 K (Ref. 78 (c) 2003, with permission from Elsevier).
FIG. 10. A scheme of ion beam synthesis of nanostructures in the immiscible matrix, showing nucleation of nanophase by supersaturation and growth of the clusters with annealing time. Coalescence leading to buried layer is also indicated.
In a minute deviation in this technique, semiconductor nanoclusters are formed by implantation of reactive ions. Nanocrystalline h-GaN is formed by N^sup +^^sub 2^ implantation in GaP at elevated temperature and subsequent annealing at high temperature.134 Similar technique is adopted later on for the formation of nanocrystalline GaN phases in GaAs by N+ implantation for analyzing detailed crystallographic (Figure 17) and optical properties.135-137 Nanocrystalline InN phase is also reported in InP showing low energy bandgap in the grown phase.138 SiC nanoclusters are reported by C+ implantation in Si substrate.139
2.4 Direct Ion Implantation Technique
2.4.1 Negative Ion Implantation
A direct negative ion implantation technique is used for the growth of nanoclusters. Negative ion implantation alleviates surface charging on insulators down to several volts and enables one to conduct accurate and efficient near-surface modification of insulators.140 Elaborate and improved work on negative ion implantation technique are reported by Kishimoto and co-authors.141 Studies are reported on Cu nanoclusters grown on varieties of substrate using negative ion implantation technique.142 Nanoclusters of Ag,143 Au,144 Si,145,146 and Ni147-149 (Figure 18a) are also grown in this technique. In addition, oxides of Ni (NiO),150 (Figure 18b) Cu (CuO,151 and Cu^sub 2^O152) nanoclusters are also formed. Superparamagnetic behavior148 is observed in Ni nanoclusters along with improved magneto-optic properties149 in comparison to that of Ni thin film. Optical gain is recorded for high quality (minimal dispersion in size distribution) Si nanoclusters grown by negative ion implantation.146 Negative ion implantations are applied to fabricate metal nanocomposites of metals and polymers, which are promising for nonlinear electronic, optical and biomedical applications.143 Authors report growth processes of Ag nanoparticle and their effects on surface properties of polymers, in comparison with that of Cu.153
FIG. 11. (a) TEM of Ag nanoclusters grown by 1.5 MeV He^sup +^ irradiation on a Ag ion exchanged sample (in a bath 0.04 mol% AgNO^sub 3^) at a fluence of 2 x 10^sup 15^ cm^sup -2^ (Ref. 82 (c) 2004, with permission from Elsevier); (b) phase formation and (c) SPR studies in Ag nanoclusters formed by similar technique of light ion implantation in Ag^sup +^ exchanged glass. (Ref. 80 (c) 2001, with permission from Elsevier).
2.4.2 Positive Ion Implantation
The most coveted technique for nanocluster formation is by direct ion implantation in insulating matrix and maximum literatures are available in this regard. A complete coverage can not be claimed in this field, as an avalanche of reports adopting this technique has appeared in last decade. However, the most of the available remarkable literature will be covered. In this technique, single element nanoclusters of Cu,154 Ag,155-157 AU;158-161 Pt,162 Co,163 Fe,164,165 Pb,166 and Se (Ref 167) are grown. Semiconductor nanoclusters with interesting PL and other optical properties of Ge,168 and Si169-175 are reported to form in insulating matrices. Nanoclusters of Zn and its oxide (ZnO) are formed by Zn+ implantation in SiO^sub 2^ and subsequent oxidation (Figure 19).176 Supersaturation of impurity atoms in the immiscible system helps in the nucleation process. A detailed TEM study addressed the long- standing question of whether the cluster nucleation is defect mediated or spontaneous in the implantation route. Atomic resolution illustrations provide a clear evidence of interstitial loops acting as nucleation sites.177
FIG. 12. Bright-field TEM image of Fe/Ag multilayers irradiated with 90 keV Ar+ ions up to a fluence of (a) 1 x 10^sup 16^ cm^sup - 2^ and (b) 7 x 10^sup 16^ cm^sup -2^. The inset shows the continuous rings in the SAED pattern corresponding to both Ag (dark) and Fe (gray) clusters. A homogeneous distribution of Fe and Ag clusters is observed with increasing fluence (Ref. 92 (c) 2004, with permission from American Institute of Physics).
A reasonable depiction for the growth of nanoclusters using 3-D kinetic Monte Carlo simulation (KMC) is reported.14 A self- organization approach to simulate cluster formation mechanism in the Oswald ripening stage is also reported (Figure 20).13,15 Implantation of Au ions into Si implanted fused quartz strongly enhances the PL intensity in the visible region.178 This effect is attributed to the enhancement of the formation of Si nanocrystals, responsible for the luminescence, by the presence of Au ions and not by ion implantation induced defects. Si nanophases are also grown in fused silica using an ion implantation technique followed by a thermal and swift heavy ion (SHI, 70 MeV Si ion) irradiation induced annealing process.179 Formation of semiconducting C, Si and metallic Fe, Co, Ni and Co nanoclusters is reported using SHI in inorganic polymers and gels.180
FIG. 13. Cross-sectional bright-field TEM images of ionimplanted silica substrates: (a) single implant Au (190 keV, fluence 3 x 10^sup 16^ cm^sup -2^); (b) sample in (a) after annealing in air 1173 K for 3 hours; (c) sequential Au (190 keV) and Cu (90 keV) at the same dose 3 x 10^sup 16^ cm^sup -2^; (d) sample in (c) after annealing at 1173 K first in H^sub 2^-N^sub 2^, 1 hour and then in air, 15 minute (Ref. 105 (c) 2002, with permission from Elsevier).
Diamond nanocrystals are formed by direct C+ implantation into fused silica followed by thermal annealing in the forming gas (4% H in Ar).181 Initially, presence of H is thought to be an important factor in the nucleation process. However, formation of diamond nanocrystals in the absence of H is also reported by C+ implantation in c-Si at elevated temperature.182 Molecular dynamic simulation results show that diamond nucleation in the absence of H can occur by precipitation of diamond clusters in a dense alpha-C matrix generated by sub-plantation. The diamond clusters can grow by thermal annealing consuming C atoms from the amohrpous matrix after nucleation. Diamond film with grains in the nanoscale is reported183 by the hot filament chemical vapor deposition method on a Si wafer. Si substartes are pre-implanted by 25 keV Si+ in order to introduce surface stress, which is believed to be one of the most important factors for low pressure diamond nucleation on the Si wafer. Fe (~3- 6 nm) nanoclusters are grown by Fe+ ion implantation in Ge [left angle bracket]111[right angle bracket] with increase in Fe concentration at high fluence that favors clustering of Fe atoms due to low solubility of Fe atoms in Ge.184 A brief review article is dedicated to the formation of metal nanoclusters in dielectric medium by low energy ion implantation with specific example of Ag in silicate glass.185 Si nanowires are reported to grow (Figure 21) on Si substrate in the catalyst driven vapor liquid solid (VLS) process using implanted Au as catalyst.186
FIG. 14. Cross-sectional TEM bright field of a silica slide sequentially implanted with Ga and N (zone A: Ga clusters, zone B: gallium oxynitride, zone C: silica matrix); histogram of the size distribution of Ga clusters is shown for zone A. (Ref. 105 (c) 2002, with permission from Elsevier).
In another growth technique, researchers have investigated the formation of Au nanocrystals due to the ion irradiation of a pre- implanted host material. The host is first implanted with Au+ at low temperatures so that the implanted material remains in solution in the near-surface region. The sample is subsequently irradiated by high energy Si+ that pass through the preimplanted region, resulting the nucleation and growth of the Au nanoclusters.187 Energy deposited due to the electronic excitation by post-implantation irradiation induces the nucleation of Au nanoclusters. Inverse Oswald ripening (growth of smaller clusters at the expense of bigger clusters) is reported for MeV Au+ irradiation in Au inclusions in SiO^sub 2^ (Figure 22).188 This observation is in agreement with the prediction that in a driven system, under appropriate ion beam and temperature conditions, the dependence of steady state solute concentration may be opposite to that prescribed in the Gibbs- Thomson relation.188
FIG. 15. Cross-sectional TEM images of a silica sample implanted with Ag and S: (a) bright-field showing the contrast between the Ag core and the Ag^sub 2^S shell; (b) high-resolution image showing the lattice planes of the Ag^sub 2^S shell (Ref. 105 (c) 2002, with permission from Elsevier).
In a significant report, control over the microstructure of the ion beam synthesized Ge and CdSe nanocrystals, embedded in a transparent sapphire matrix, is achieved using substrate amorphization and recrystallization (Figure 23).189 Controlling of size, orientation, and lattice parameter of semiconductor nanocrystals are achieved by manipulating the substrate microstructure using ion beam. This process induces changes in impurity solubility, crystal symmetry anin turn exert a profound influence on the microstructure of the embedded precipitates. Moreover, this approach predicts to get control over virtually any type of precipitate (e.g., metals, insulators or magnetic clusters) and epitaxial thin films. FIG. 16. High-resolution image of a CdS precipitate in SiO^sub 2^ glass. There is a light-contrast feature in the center of the nanocrystal. The specimen from which this image was obtained was implanted to obtain a local concentration of 5.3 x 10^sup 21^ cm^sup -3^. Panels (b) and (c) are zero-loss and low- loss electron energy loss spectroscopic (EELS) images and (d) is a t- over-lambda map (see text in Ref. 129). Panel (e) shows a mass thickness (intensity) profile across the large precipitate in (d) (Ref. 129 with permission from authors).
While reporting other nanostructures, nanometer sized carbon onions and nanocapsules are formed by C+ implantation in Cu.190 Ge nanowires are also reported in the pre-grooved SiO^sub 2^ coated Si surface.191 A novel model of surface topological evolution is described to study the Ge redistribution in the V-groove during ion irradiation.
3. DEFECT DYNAMICS IN NANOCLUSTERS
As introduced earlier, defect is an indispensable part while studying materials under energetic ion irradiation. Instead of the general conception of destruction of materials using ion beam processing (e.g., ion cut by FIB, deformation of structures under energetic irradiation, transmutation of biological cells under prolonged exposure to radioactive elements or compounds), I will rather start on a constructive note on crystallization and phase formation under controlled irradiation of specific crystallographic structure. Irradiation process can even modify chemical bonds in order to form nanostructures without physical desorption of any elements. This is different from IBICVD, in which nanocluster are formed (section 2.1) with desorption of lighter component from the host matrix. In fact, knockon displacement of atomic species helps defects to be released from the nanostructures and is termed as "dynamic annealing" (defect annihilation), which will be discussed in this section. As crystallization and phase formation goes hands in hand, both of them will be addressed under the heading "defect nucleation and phase formation" with the common understanding that nucleation of defects originates the second phase formation.192 In the ion beam process, dynamic annealing leads to further crystallization of the newly formed phase to realize the complete phase transition.
Amorphization is also an important state of materials both from fundamental as well as technological interests. Hence, this process will be also discussed in terms of long-range transportation of atomic species under RIS.12 Amorphization originates either homogeneous192,193 or heterogeneous194 ways in a nucleating model. In a non-nucleating model, planar amorphization is reported.195 Resistance to chemical effect and general degradation is the key technological issue in amorphous materials. The state of materials even beyond amorphization will be covered. Supersaturation of defects nucleate voids in the material at extreme irradiation conditions. Bubbles are also reported to form during the accumulation of gaseous spices either by direct implantation in materials or in the process of disintegration in a compound exposed to high irradiation fluence. Accumulation of these voids or bubbles leads to deformation of materials.
3.1 Role of Electron Irradiation
3.1.1 Defect Nucleation and Phase Formation
Desorption of energy in the electron irradiation process is reported to anneal amorphous nanostructures with the interplay of defects. a-SiO^sub 2^ nanostructures are reported to transform into c-Si by 200 kV electron irradiation at ambient conditions. At first, a-SiO^sub 2^ is transformed into a-Si by valence electron ionization leading to disruption of Si-O bonds under electron irradiation. The crystallization of a-Si occurs (Figure 24) with beam heating and knockon displacement of Si atoms.196 Structural evolution of Si clusters under electron irradiation is investigated in detail by HRTEM analysis. Si nanocrystals show an unusual structure and experience a transformation from 2-D order to a crystal. The transformation is mainly due to the increase in cluster size where knockon displacement helps the defects to escape from the nanocrystals.197 Growth and crystallization of a-Ge nanoclusters, grown by direct ion implantation in SiO^sub 2^ matrix, are reported by electron irradiation with progressive fluence and flux.198 Ge nanoclusters are found in a complete crystalline state at a high flux of 150 A.cm^sup -2^. Electron energy is not sufficient to displace Ge atom in the electro-nucleus collision. Dominant mechanism of recrystallization is the diffusion to the amorphous- crystalline interface of the defects produced by elastic displacements both in the crystalline and in the amorphous region.199 Twinned and multiply twinned nanoclusters are reported at very high fluxes. Density and frequency of the thermal spike are suggested for the observed multiple twining. Transformation of nanobelt of PbO^sub 2^ to Pb with an intermediate PbO phase under TEM electron irradiation is reported in the high vacuum condition.200
FIG. 17. Cross-sectional TEM micrographs of a GaAs sample implanted with 3 x 10^sup 17^ N+ cm^sup -2^. (a) As-implanted and (b) the corresponding SAED pattern; (c) annealed at 1123 K for 10 min and (d) the corresponding SAED pattern. Diffraction spots from the GaAs zone axis are marked by black dots. Note that the diffuse rings in (b) are due to the amorphous layer and precipitates in (a), whereas the diffraction rings (indexed with four-digit numbers) in (d) are due to reflections from the hexagonal GaN (alpha- phase) layer in (c). Note also that the reflections corresponding to the cubic GaN (beta-phase) zone axis (indicated by arrows) are due to the precipitates below and above the alpha-GaN layer (Ref. 136 (c) 7995, with permission from American Institute of Physics).
Evolution of defects created by electron irradiation also leads to the formation of different nanostructures, sometimes leading to a phase transition. Bamboo like long hollow nanopipes in epitaxial (epi-) GaN thin films is found to evolve under electron irradiation.201 The morphological evolution is proposed in the following steps. In the beginning, roughening of the lateral surface of the nanopipe appears because of differences in surface energy of crystallographic planes. Then, after several minutes of irradiation, pinholes connected by a c-screw dislocation are nucleated in place of nanopipes leading to pinch off. At the last, the pinholes move away from their nucleation position, and their growth occurs due to an Oswald ripening process. The mass rearrangement that allows this process is provided by the point defects created during electron irradiation. The formation of epitaxial nanotubes (nanoarches) on the surface of hexagonal BN (H-BN) during electron irradiation is reported.202 In addition to implications in terms of understanding fullerene based structures, it is suggested that these act as the nucleation sites for cubic BN (c-BN) growth and may lead to improved film growth. Electron irradiation of h-BN causes the edges of the graphite like sheets of BN to curl around and form nanoarches, or half-nanotubes capping the ends of the sp2-bonded sheets. It is suggested that the sharp radius of curvature at the tips of the nanoarches allows them to act as sp3 nucleation sites for the cubic phase. The most interesting example of phase transformation under MeV electron irradiation is found in case of graphitic carbon onions to diamond at low pressure and moderate temperature (Figure 25a).203 The diamond crystals grow with further irradiation until the graphitic onions have wholly transformed to diamond. It is concluded that the conversion of the graphitic structure to diamond starts at high pressure and proceeds at decreasing, possibly even at zero pressure. Only single interstitials and vacancies are assumed to be produced by the electron irradiation (no cascade damage) and exchange of atoms between diamond and graphite is due to interstitials produced directly at the interface (Figure 25b,c). The destabilization of graphitic structures with respect to low- pressure growth of diamond is due to the large difference in the cross sections for irradiation induced displacements of C atoms in diamond and graphite. In a contrasting result, the transformation of diamond nanoclusters into onion like C is observed during electron beam irradiation inside an ultrahigh vacuum transmission electron microscope at 300 kV.204 Small nanoclusters (~5 nm) of diamond changed into carbon onion in short duration. Relatively large (~20 nm) diamond clusters change to graphite and not to onion like structure. The reason may be that graphite layers from the other diamond clusters affect the generated graphite layers around the large clusters since the speed of phase changes for the large clusters is slow.
FIG. 18. A cross-sectional transmission electron microscopy image of (a) Ni nanoclusters in SiO^sub 2^ implanted with 60 keV Ni- to 5 x 10^sup 16^ cm^sup -2^ (Ref. 147 (c) 2004, with permission from Elsevier), and (b) NiO clusters after oxidizing under O2 gas flow at 1073 K for 1 hour. Ni and NiO nanoparticles are observed as black dots in a surface layer (Ref. 150 (c) 2004, with permission from American institute of Physics). A structure on the sample surface is due to glue.
The experimental observation of large superheating effects in Sn and Pb clusters encapsulated in graphitic carbon onions is reported.205 Supercooling are only observed for Sn clusters. The pressure (~2-3 GPa) inside the carbon onions resulting from the electron bombardment in the electron microscope plays a major role in defining the of the observed superheating. Closed shell C nanostructures, such as carbon onions act as self-contracting high- pressure cells under electron irradiation. This report, to encapsulate clusters of metals inside carbon onions, combined with the possibility of exerting moderately large pressures on the encapsulate opens a new chapter of material research. Controlled irradiation of multiwalled nanotubes (MWNTs) of C can cause large pressure buildup within the nanotube cores that can plastically deform, extrude, and break solid materials that are encapsulated inside the core (Figure 26).206 Atomistic simulations show that the internal pressure inside nanotubes can reach values higher than 40 GPa. Thus nanotubes can be used for extruding and deforming hard nanomaterials and for modifying their properties, as well as templates for the study of individual nanosized crystals under high pressure. Electron beam induced surface quasimelting of Co granular nanowires is studied using a HRTEM.207 Morphological change in the Co granular nanowires is quite different from that in a Co nanocluster in terms of structures and phase transitions as revealed from the time evolution of electron microscope images as a function of the irradiation time. Allotropic beta to a transition, inhibited in the Co nanoclusters, is observed to proceed in the Co nanowire through the lattice softening of face centered cubic (fee) along (111) facets without dimensional collapse. Fragmentation of submicron sized highly graphitic carbon fiber (HGCF) to graphitic nanocrystall ites is reported under electron irradiation both at low temperature (90-120 K), room temperature and above (Figures 27a- e).208 Sharp diffraction spots corresponding to single crystalline HGCF transform to halo ring pattern with electron irradiation, suggesting the fact that longrange order within the basal planes is lost, and at the low temperatures it is lost more quickly than at room temperature. With other radiation effects remaining similar,  diffraction pattern is transformed to complete halos much more quickly than at room temperature irradiation. The buckling of basal plane and the lattice dilation in the c-direction, similar to the results obtained at room temperature, suggest the local formation of non-hexagonal atomic rings (Figures 27a, b). On the analogy of observations made in fullerenes, it is by the displacement damage incorporated with electronic excitations. These suggest that the displacement damage in the complete damaging process plays two types of roles, the fragmentation of the crystals with random orientations and the reconstruction of nonhexagonal atomic rings in combination with electronic excitations. At lower temperatures the former proceeds faster because of the generally slower recovery rate of the displacement damage, while the latter exhibits less temperature dependence because the process must involve electronic excitations. Irradiation above room temperature (~650 K) shows sharp diffraction Deby-Scherrer rings indicating the restoration of long range order in the nanocrystallites with internal hexagonal ring network (Figures 27c-e).
FIG. 19. Cross-sectional TEM images of SiO^sub 2^ samples implanted with Zn+ ions of 60 keV to a fluence of 1.0 x 10^sup 17^ cm^sup -2^, (a) in as-implanted state and after annealing in oxygen gas for 1 hour at (b) 873 K, (c) 973 K, and (d) 1173 K (Ref. 176 (c) 2006, with permission from American Institute of Physics).
Electron irradiation in carbon nanotube (CNT) is also studied extensively. Threshold energy for ballistic damage in single walled nanotubes (SWNTs) of C is determined by electron irradiation studies.209 The threshold energies are be attributed to anisotropic ejection. It is not due to beam heating or isotropic ejection as expected earlier. The production and migration of C interstitials in CNTs under electron irradiation is studied.210 It is demonstrated that the threshold for displacing C atoms in MWNTs and the defect production rate depend on the nanotube diameter because of the curvature induced strain in the nanotube atomic network. It is also shown that CNT under electron irradiation shrink by a loss of atoms inside the tubes and by diffusion of interstitials through the inner hollow in the axial direction (Figures 28a-d). Therefore, one can consider CNTs as pipes for the effective transport of interstitial atoms. One can further expect that foreign atoms or molecules are also highly mobile inside the hollow cores so that nanotubes appear ideally suited as pipelines on the atomic or molecular scale (Figures 28e-h). As another example of atomic movement under electron irradiation, bundles of SWNTs can be cut with a focused electron beam with the lateral precision is of ~1 nm.211 However, it is difficult to cut a second gap when the inner core of the tubes is blocked and interstitials cannot vanish. The interstitials improve the mechanical properties of the tubes by annealing defects by dynamic annealing. The experiment provides information about the migration behavior of interstitials, moving preferentially inside the tubes so that they vanish from the region under the beam. It is thought that these interstitial atoms can possibly be ejected from the tube cap when applying a voltage, so that they could be used as source of atoms (as a pen from the SWNT tip or ejecting an atom as pumping action).
The coalescence of SWNT under electron irradiation at high temperature in TEM is reported.212 The merging process is investigated using tight binding molecular dynamics (TBMD) and Monte Carlo simulations at the atomic level. Vacancies induce coalescence via a zipper like mechanism, which imposes a continuous reorganization of atoms on individual tube lattices along adjacent tubes. Low frequency of occurrence of this event is due to coalescence seems to be restricted to tubes with the same chirality. High temperature (~1073 K) electron beam (1.25 MeV, 10 A*cm^sup - 2^) "nanowelding" of crossing SWNTs tubes junctions is reported (Figures 29a,b).213 This technique is proposed to be an alternative to possible chemical functionalization. The nanotube junctions are created via vacancies and interstitials (crosslinking of dangling bonds), induced by the focused electron beam promoting the formation of internanotube links. TBMD