synthesis of inorganic fullerene-like nanostructures by

DOI: 10.1002/ijch.201000056

Synthesis of Inorganic Fullerene-like Nanostructures byConcentrated Solar and Artificial LightMoshe Levy,[a] Ana Albu-Yaron,[a] Reshef Tenne,*[a] Daniel Feuermann,[b] Eugene A. Katz,[b, c]

Dotan Babai,[b] and Jeffrey M. Gordon*[b, d]

1. Introduction

The use of concentrated sunlight to drive chemical reac-tions has been investigated extensively.[1–5] The high at-tainable flux density provides reactor conditions that arefar from equilibrium, allowing high-temperature reactionswhich cannot otherwise be attained easily. The chief moti-vation has been the synthesis of storable chemical fuels.Highly concentrated sunlight was also applied to the pro-duction of high-end nanomaterials such as single- andmulti-wall carbon nanotubes,[6–8] carbon fullerenes,[9–11]

nested carbon fullerenes,[12] oxide nanoparticles,[13] andzinc powder.[14]

Different intense light sources have been used to drivechemical reactions, most notably focused lasers. Thephoto-induced generation of nanoparticles has been ex-plored for more than two decades, with much of the workfocused on oxide,[15] metallic,[16] and semiconducting[17]

nanoparticles.In the present study — an outgrowth of previous inves-

tigations[18–20] — the objective is different: to exploit theextreme conditions that can be created in reactors drivenby highly concentrated sunlight to generate nanoparticleswhich cannot otherwise be produced, or can only be ob-tained with considerable difficulty.

The two most relevant interactions in two-dimensional(2D, layered) nanomaterials are the strong chemical(mostly covalent) bonds in the molecular sheet, and theweak van der Waals interactions responsible for thestacking of the layers. The latter interaction grows insub-stantial at the high temperatures that enhance the proba-bility that thermal fluctuations will bend, shear, and foldmolecular sheets into closed-cage nanostructures.

The hypothesis underlying the experiments coveredhere is that high flux density (at adequately high flux)allows reactions to be driven far from equilibrium intoenergy landscapes not accessible by conventional ther-

mally-driven reactions, including unique photonically-hotannealing environments created by the thermal radiationemitted from the irradiated precursor material. Solarablation of 2D compounds like MoS2 was investigatedwith the aim of synthesizing hollow closed nanoparticles,i. e. , inorganic fullerene-like structures and nanotubeswhich cannot be obtained by common high-temperaturesyntheses.[21, 22] As will be shown below, the extreme con-ditions (temperatures apparently exceeding ~2700 K)may lead to the formation of singular nanostructuressome of which were not hitherto observed.

The present paper summarizes earlier studies in whichinorganic fullerene-like and nanotubular structures weregenerated by highly concentrated non-coherent (solarand lamp) light. Also, new data for exfoliated MoS2 and

Abstract : A track record of generating novel fullerene-likeand nanotubular inorganic nanostructures from Cs2O,SiO2-x, WS2, and MoS2 by photothermal ablation with highly

concentrated sunlight and ultra-bright lamp light is re-viewed, and augmented with new results for exfoliated MoS2

and carbon as well as carbon nanotubes.

Keywords: concentrators · fullerene-like · nanoparticles · nanostructures · solar

[a] M. Levy, A. Albu-Yaron, R. TenneDepartment of Materials and Interfaces, Weizmann Institute ofScience, Rehovot 76100, Israelphone: +972 (0)8 9342394fax: +972 (0)8 9344138e-mail: [email protected]

[b] D. Feuermann, E. A. Katz, D. Babai, J. M. GordonDepartment of Solar Energy and Environmental Physics, JacobBlaustein Institutes for Desert Research, Ben-Gurion Universityof the Negev, Sede Boqer Campus 84990, Israelphone: +972 (0)8 6596923fax: +972 (0)8 6596921e-mail: [email protected]

[c] E. A. KatzThe Ilse Katz Institute for Nanoscale Science and Technology,Ben-Gurion University of the Negev, Beersheva 84105, Israel

[d] J. M. GordonThe Pearlstone Center for Aeronautical Engineering Studies, De-partment of Mechanical Engineering, Ben-Gurion University ofthe Negev, Beersheva 84105, Israel

Isr. J. Chem. 2010, 50, 417 – 425 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 417

Review

carbon, as well as carbon nanotubes, which support thesuggested working hypothesis are presented. The threeexperimental apparati and affiliated results are describedin the chronological order in which they were developed,and hence in the order in which lessons learned aboutdriving nanomaterial reactors with concentrated non-co-herent light were learned: (1) solar fiber-optic mini-dishconcentrators that achieved ~4,000 suns on the precursormaterial (1 sun = 1 mW/mm2); (2) near-field optics thatreconstitute the immense power density of the plasmasource of ultra-bright continuous short-arc dischargexenon lamps on a remote target, and produced up to

Moshe Levy received his M.Sc. inChemistry at The Hebrew University ofJerusalem in 1952 and his Ph.D. at theCollege of Environmental Science andForestry, State University of New York,Syracuse, in 1955. He served as Lecturerin Physical Chemistry at the Technion,Haifa from 1957 to 1960. From 1962 hehas been at the Weizmann Institute ofScience, Rehovot, serving as Head ofthe Department of Plastics Researchfrom 1977 to 1983 and as Emeritus Pro-fessor since 1993. He served as Presi-dent of the Israel Polymer and PlasticsSociety, and as Editor of Chemistry inIsrael, the Bulletin of the Israel Chemical Society. His main contribu-tions were the discovery of “Living Polymers” (1956), “Dormant Poly-mers” (1962), and “Chemical Storage of Solar Energy via a closed-loop Chemical Heat Pipe” (1993).

Ana Albu-Yaron (Ph.D. in InorganicChemistry, Hebrew University of Jerusa-lem) is a materials scientist. She washead of the Materials Research Lab, Ag-ricultural Research Organization, VolcaniCenter, Bet-Dagan 1972–2001; Sr. Lec-turer in Inorganic chemistry, 1978–1987,Hebrew University, Rehovot Campus;visiting scientist, Universite Pierre etMarie Curie, Paris, 1974; Department ofMaterials, Oxford University, 1978–79,83–84, 87–88; Universite Paris Sud atOrsay and CNRS Meudon 1993, 2000.Since 2001, she joined as advisor to theDepartment of Materials and Interfaces,Weizmann Institute of Science, where her current research focuses onthe synthesis of novel nanomaterials — quantum dots, inorganic ful-lerenes, nanotubes (including doped or filled nanotubes) — and thestudy of their structure, mainly by HR-TEM.

Reshef Tenne earned his Ph.D. in 1976in physical chemistry at The HebrewUniversity of Jerusalem. He joined theWeizmann Institute in 1979, working inthe field of photoelectrochemistry, andwas promoted to professor in 1995. Hisinterests are focused on the synthesisand study of nanotubes and fullerene-like nanoparticles of layered com-pounds, like WS2, which he discoveredin 1992. He is recipient, among others,of the Materials Research Society (MRS)Medal; the Israel Chemical Society Prize(2008), and the European Research So-ciety (ERC) Advanced Research Grant(2008).

Daniel Feuermann is associate professorat Ben-Gurion University of the Negev.His current research interests are in theapplication of imaging and non-imagingoptics to illumination and irradiationproblems in general, and in high-con-centration photovoltaic power produc-tion in particular; characterization andtesting of high-concentration photovol-taic cells; high-concentration solar fur-nace for high temperature (>2000 K) in-organic nanomaterial synthesis.

Eugene A. Katz received a M.Sc. in ma-terials science (1982) and Ph.D. in solidstate physics (1990) at Moscow Instituteof Steel and Alloys. In 1995, as a visitingscientist at the National Solar EnergyCenter of Ben-Gurion University of theNegev (BGU), he started to investigategrowth, structure, and photoelectricalproperties of fullerene thin films. In1997, he joined BGU’s Blaustein Insti-tutes for Desert Research, and has beenworking in the Department of SolarEnergy and Environmental Physics eversince. In 2006, he became a member ofBGU’s Ilse Katz Institute for NanoscaleScience and Technology. His research interests include photovoltaicsbased on non-traditional semiconductors (fullerenes, carbon nano-tubes, conjugated polymers), photovoltaic characterization of concen-trator solar cells, and solar synthesis of nanomaterials.

Dotan Babai received his B.S. in Physicsand B.Sc. in materials engineering fromBen-Gurion University in 2004. He iscurrently a M.Sc. student at the JacobBlaustein Institutes for Desert Research,Ben-Gurion University, Sede BoqerCampus. His thesis research, supervisedby Prof. Jeffrey Gordon, is about the de-velopment, construction, and characteri-zation of an ultra-high flux concentra-tion system, as a tool for utilizing highlyconcentrated solar radiation as a diag-nostic probe, as a characterization toolfor photovoltaic cells, and for drivingthe synthesis of nanomaterials.

The principal research and teaching in-terests of Jeffrey Gordon — professor atBen-Gurion University of the Negev(BGU) — subsume the physics andcharacterization of ultra-efficient solarcells, novel photovoltaic concentrationwith miniaturized optics, radiative trans-fer at the thermodynamic limit, and thelight-assisted synthesis of nanomateri-als. Currently, he supervises 7 graduatestudents and 2 post-doctoral fellows.He is among the founding generation(1970s) of BGU’s Blaustein Institutesfor Desert Research at the university’sSede Boqer Campus, 50 km south ofBeerSheva.

418 www.ijc.wiley-vch.de � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Isr. J. Chem. 2010, 50, 417 – 425

Review M. Levy et al.

http://www.ijc.wiley-vch.de

~7,000 suns on the target powder; and (3) a solar furnacewith a peak target irradiance of ~15,000 suns.

All the transmission electron microscopy (TEM) analy-ses reported below were performed by transferring theproducts from the reactor ampoules to TEM grids, wherethey were examined with a Philips CM-120 120 kV TEMand a FEI Tecnai F-30 300 kV high-resolution TEM.

2. Solar Fiber-Optic Mini-Dish Concentrators andAssociated Nanomaterial Syntheses

2.1. Concentrator Optics

Figure 1 shows the first generation of solar concentratoroptics used for the experiments. Although during clear-sky mid-day periods up to 8 W were measured at thedistal fiber tip (a nominal flux concentration of~10,000 suns),[18–20, 23] the inherently diverging nature oflight delivery resulted in dilution of the power density onthe precursor material inside the quartz ampoule reactor,such that irradiance values on the precursor powderswere typically only ~4,000 suns. Irradiation periods wereusually several minutes.

These solar-driven reactor conditions generated a ple-thora of nanostructures from Cs2O, SiO2-x, and MoS2,which will now be reviewed.

2.2. Dicesium Oxide Fullerene-like Nanoparticles

The Cs–O phase diagram was investigated to some extentfor more than 100 years, and more intensively in thesecond half of the 20th century.[25] In particular, the syn-thesis of Cs2O and its structure were elucidated inrefs. [26,27]. Cs2O has an anti CdCl2 (layered) structure,i. e. , R3 m symmetry and unit cell parameters a = b =4.256 � and c = 18.990 � (Figure 2a). In each molecularsheet, the layer of oxygen atoms is sandwiched betweentwo layers of cesium atoms in an octahedral arrangement.

Due to its highly polarizable character, it exhibits a lowwork function and electron affinity (~1 eV). Coatingswith a 2 : 1 Cs : O ratio confer a low threshold voltage andhigh sensitivity to electron emission devices, includingphoto-cathodes and negative electron-affinity devices.[28]

Cs2O reacts readily with ambient oxygen, CO2, andwater, and hence must be handled and analyzed underhigh vacuum conditions. In particular, the study of cesiumoxides using TEM and its auxiliary techniques, like elec-tron diffraction, energy dispersive X-ray spectroscopy(EDS), and electron energy loss spectroscopy (EELS) re-quires a special means to transfer the sample safely.

To that end, a special environmental chamber wasmounted on the TEM entry port (Figure 2b).[29] Usingfirst laser,[29] and more recently solar ablation,[18] hollow

Figure 1. Our fiber-optic mini-dish concentrator for first-generation solar ablation.[18–20, 23] (a) Schematic of the mirrored paraboloidal dish200 mm in diameter, with a small flat mirror that images the sun into the upward-facing tip of a high-transmissivity optical fiber (typically7–10 m long) that guides concentrated solar radiation to an indoor laboratory where direct irradiation of the powder inside a sealed quartzampoule (evacuated or filled with helium) is performed. Input power is moderated by an iris that preserves the broad angular distributionof delivered sunlight (numerical aperture of 0.66) inherently required for achieving high flux concentration.[24] (b) The dual-axis trackingunit in the field. The optical fiber pointed at the camera was subsequently threaded into the lab for the described experiments. (c) Photo-graph during irradiation of MoS2 powder inside a sealed evacuated ampoule.

Figure 2. (a) The lattice of Cs2O (anti CdCl2 structure with R3 msymmetry) with the black spheres in the center of the three-layersandwich representing oxygen atoms and the larger grey outerspheres representing cesium atoms. (b) Photograph of the environ-mental chamber attached to the TEM for analysis of air-sensitivesamples. This chamber was attached to the CM120 TEM used toanalyze the fullerene-like Cs2O nanoparticles.[18, 29]

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Synthesis of Inorganic Fullerene-like Nanostructures by Concentrated Solar and Artificial Light


closed nanoparticles of Cs2O were synthesized by irradiat-ing 3R-Cs2O powder sealed in an evacuated quartz am-poule. The yield was meager (e.g., at least two fullerene-like Cs2O nanoparticles could be viewed and fully ana-lyzed in each TEM session) but reproducible (Figure 3).The structure and composition of these unique nanoparti-cles were confirmed by electron-diffraction, EDS, andEELS analysis.

Some of the analyzed nanoparticles were undamagedand showed the usual 0.633 nm interlayer spacing. How-ever, the nanostructures shown in Figure 3 show signs ofdeterioration, especially the outer layers which are gradu-ally exfoliating. This phenomenon resulted from the diffi-culty in maintaining a perfectly inert atmosphere withinthe environmental chamber, which led to partial reactionwith the residual water/air during the transfer of thenanoparticles from the opened ampoule to the TEM grid.In contrast, bulk Cs2O platelets degrade rapidly in thiskind of environment. The metastable character of suchnanoparticles notwithstanding, producing a closed hollowstructure confers a kinetic stabilization that appreciablyretards the reaction with the atmosphere. As is evidentfrom the TEM analysis, the reaction starts at the weakpoints of the nanoparticle, i. e. , kinks and cusps in theclosed structure, which provide the nucleation sites.

2.3. Silicon Dioxide Nanowires and Nanospheres

A vast literature exists on the synthesis of Si and SiO2-x

nanowires, particularly by the vapor–liquid–solid pro-

cess.[30–32] However, SiO2-x nanostructures synthesized bystarting with quartz, and via a nominally photothermalprocess, had not been reported previously, most likelydue to the high thermal stability of this compound.

Therefore it was somewhat surprising when solar abla-tion experiments on MoS2 powder in a sealed quartz am-poule produced substantial amounts of amorphous SiO2-x

nanowires and nanospheres.[19] Subsequently, solar abla-tion of mixtures of ground quartz and powdered MoS2

(MoS2 constituting the black bystander needed to absorbthe solar radiation and heat the mixture) yielded copiousquantities of the same genres of SiO2-x nanostructures.(This process can also produce volatile SiO, which pro-motes the high-temperature formation of amorphousSiO2-x nanowires.)

Figure 4a shows examples of tortuous SiO2-x nanowires,20–50 nm in width and up to 5000 nm long, seeminglyrooted in the MoS2 crystallites. Experiments in whichMoS2 was replaced by WS2 or graphite (both black) pro-duced the same results, indicating that any possible cata-lytic effect of the black bystander powder is probably in-significant. SiO2-x nanospheres 200–300 nm in diameterwere also found in the vicinity of the MoS2 crystallites(Figure 4b), and multi-wall fullerene-like MoS2 nanoparti-cles were simultaneously generated in separate regions(not shown).

2.4. Silicon Nanowires

Silicon nanowires have been obtained[33] by partial reduc-tion of SiO2 into volatile SiO, and subsequent dispropor-tionation into Si and SiO2-x. Pursuing this direction, weperformed solar ablation on powdered 1 :9 mixtures ofMoS2 :SiO, and indeed produced a high density of ran-domly distributed long, winding Si nanowires with uni-form diameters of 10–25 nm and lengths from hundredsto thousands of nm (Figure 5).[19] (Because SiO is trans-parent to the solar spectrum, MoS2 was introduced as ablack bystander, as in the prior production of SiO2-x nano-structures.)

Figure 3. TEM images of nested fullerene-like Cs2O nanostructuresachieved by solar ablation of pure crystalline Cs2O in vacuum.[18] (a)Quasi-spherical, 20–30 nm in diameter, composed of 14 layers. (b)Closed polyhedra, 25 nm in diameter, comprising 12 layers. Notethe hollow core in parts (a) and (b). (c) Smaller spherical structure,20 nm in diameter, constituting 12 layers and exhibiting a smallercore. (d) and (e) show nanoclusters where the outermost layersappear to have been damaged by reaction with the ambient at-mosphere. (All scale bars are 10 nm).

Figure 4. Amorphous SiO2-x nanostructures obtained by solar abla-tion of SiO2 powder in a quartz ampoule containing MoS2

powder.[19] (a) Nanowires. (b) Nanospheres.




2.5. Molybdenum Disulfide Fullerene-like and NanotubularNanostructures

There is an extensive literature on the experimental andtheoretical basis for MoS2 forming fullerene-like and tub-ular nanostructures,[21,22, 34–37] and Figure 6 reviews the sali-ent relevant features of the layered structure of MoS2.The rich multiplicity of MoS2 nanostructures synthesizedwith high-temperature chemical and laser ablation meth-ods prompted exploration of the solar route.

Figure 7 shows representative TEM images from thesolar ablation of MoS2 powder.[19] Figure 7a and b illus-trate multi-wall nanotubes typically 30 nm in diameter

and reaching a length of a few microns. Note the perfectlystraight walls, which suggest that they are essentiallydefect-free. Multi-wall fullerene-like nanoparticles ofMoS2 (e. g., Figure 7c) were obtained in substantialamounts, though with quite large size and shape distribu-tions.

In rare cases, single-wall nanotubes and fullerene-likestructures were obtained (Figure 8). In fact, single-wallnanostructures had not been obtained spontaneously byany other thermal reaction,[21,22, 34] which is indicative ofthe importance of high temperatures (where the van derWaals interaction becomes less relevant) and rapidquenching of the product.

3. Lamp Ablation

3.1. Near-Field Optics

The solar ablation achievements described in Section 2were tempered by (a) the optically diverging character offiber-optic light delivery, limiting the achievable irradi-ance on the reactant powder inside the ampoules to~4,000 suns, and (b) working limitations imposed by theephemeral nature of direct solar beam radiation. Giventhe recent availability of commercial lamps with source

Figure 5. Si nanowires obtained by solar ablation of a SiO/MoS2

mixture.[19] In the high-resolution TEM inset, the arrows indicate alattice spacing of 0.31 nm, consistent with the [111] reflection of Si,and the outer surface of the nanowire is covered with a thin passi-vation film of SiO2-x.

Figure 6. Schematics of the MoS2 structure. (a) Mo atoms are intrigonal prismatic coordination, with MoS2 layers stacked along thec-direction. (b) Single MoS2 sheet viewed from the [001] direction.The bulk Mo and S atoms are 6-fold and 3-fold coordinated, re-spectively, but are only 4-fold and 2-fold coordinated in the nano-cluster, which destabilizes the planar configuration and favors theformation of hollow, closed, fullerene-like, and tubular nanostruc-tures. (The same structural traits also pertain to WS2.)

Figure 7. TEM images of representative MoS2 nanoparticles ob-tained by solar ablation of bulk MoS2 powder.[19] (a,b) Nanotubes.(c) Fullerene-like closed-cage structure. All scale bars are 10 nm.

Figure 8. TEM pictures of (a) single- and (b) double-walled MoS2

nanostructures, as well as (c) single-walled nanotubes, all producedby solar ablation.[19]




radiance values rivaling those of the solar surface, near-field optics for lamp ablation in nanomaterial reactorswas developed, aiming for results comparable to those ofthe solar-driven reactors. A dual-mirror unfolded aplana-tic converging optic (Figure 9) was designed, character-ized,[38] and found capable of delivering a peak irradianceof 7 W/mm2 on a 1 mm2 target.

3.2. Nanostructures by Lamp Ablation

Lamp experiments could basically replicate the sametypes of nanoparticles portrayed in Section 2. One addi-tional example (Figure 9c) shows a multi-wall nanotubeproduced from WS2 powder. The remarkably straightwalls are indicative of the highly crystalline (defect-free)nature of the nanostructure.

Compared to the earlier solar ablation system, thehigher target power densities (and hence hotter tempera-tures and annealing conditions) generated in the lamp-driven reactor were expected to improve the likelihoodof generating additional classes of nanostructures, be-cause distinctions among energy landscapes are progres-sively diminished as temperature increases.

Indeed, two examples from lamp ablation were particu-larly prominent: (1) MoS2 nano-octahedra at the funda-mental smallness limit (Figure 10), the theoretical and ex-

perimental significance of which were reviewed inrefs. [35–37], and (2) carbon nanotubes generated withoutcatalysis (Figure 11). Previously, the kinds of MoS2 nano-octahedra shown in Figure 10 had only been observed intwo studies based on pulsed laser ablation.[35–37]

Figure 9. Lamp ablation generation of nanomaterials, driven by reconstituting the immense flux density of the xenon discharge lamp’splasma-arc source at a remote reactor with near-field aplanatic converging optics.[38] (a) Schematic side view. The lamp includes a smallhemispherical mirror that recycles its emission from the hemisphere facing away from the optical system back into the radiant plasma. (b)Photograph of the assembled system, with an entry and exit diameter of 190 mm. The lamp and the reactor ampoule are at the right andleft sides of the optic, respectively. The numerical aperture is 0.93 for both the entry and exit of the optic. (c) TEM picture of part of a WS2

nanotube obtained by lamp ablation. The scale bar is 10 nm.

Figure 10. TEM images of an assortment of MoS2 nano-octahedra(indicated by the arrows) and other nanostructures generated frompure MoS2 powder by lamp ablation. The magnified inset at thelower right shows an individual nano-octahedron composed oftwo MoS2 walls 0.62 nm apart.




Reports of the photothermal production of carbonnanotubes from pure graphite, without catalysis, havebeen rare. Products of the sort in Figure 11 invariably re-quire the initial sublimation of graphite, meaning reactortemperatures of at least ~2700 K. An independent confir-mation of this estimate derives from the production ofMoS2 nano-octahedra appearing to necessitate a vaporpressure of Mo that corresponds to a temperature of atleast ~2700 K.

4. Solar Furnace for Nanomaterial Production atUltra-High Irradiance

4.1. Solar Furnace Optics

It was surmised from the first-generation solar and lampablation optics that progressively higher flux concentra-tion amplifies the probability of forming and detecting anassortment of novel nanostructures — more specifically,those in which the weak van der Waals interaction in lay-ered materials becomes less relevant. Under these condi-tions, rapid quenching at ultra-hot non-equilibrium condi-tions might produce exfoliated nanosheets. This prompteddevelopment of the solar furnace illustrated in Figure 12for which the attainable peak irradiance is~15,000 suns.[39]

4.2. Exfoliated MoS2 and Carbon

After nanomaterial results akin to those obtained in thesolar fiber-optic and lamp concentrators were reproducedby operating the solar furnace below its maximum attain-

able flux concentration, solar ablation at ~15,000 sunswas conducted on pure MoS2 and graphite powders invacuum.

A variety of chemical strategies have been devised tosynthesize graphene-like sheets of MoS2,

[40,41] most nota-bly intercalating lithium ions into MoS2 crystallites andexfoliating by immersion in water.[42] Here, the ablationof pure MoS2 powder in the solar furnace led to extensiveexfoliation (Figure 13a–b): MoS2 foils, 1–3 molecularlayers thick, were obtained in large yields (but only overa limited sample region). The same experiment with puregraphite powder yielded basically double-walled exofilat-ed graphene-like arrays (Figure 13c).

Figure 11. TEM image of a representative multi-wall carbon nano-tube from lamp ablation of pure graphite powder (no catalyst) invacuum. The scale bar is 10 nm.

Figure 12. Solar furnace (based on converging Gregorian-tele-scope optics).[39] Sunlight is reflected into the lab from a dual-axistracking outdoor mirror, reflected upward by a flat tilted mirror (A),to a parabolic dish (B) the focus of which lies slightly below thetilted mirror. An elliptic dish (C) of high numerical aperture raisesthe flux concentration to ~15,000 suns at its proximate focus F(high numerical aperture is a necessary condition for achievinghigh concentration[24]). The reactor ampoules were introduced tothe focus F from the right and moved horizontally by about 1 mmper min in order to irradiate as much of the precursor powder aspossible. (a) Schematic. (b) Side photograph.

Figure 13. TEM images of exfoliated graphene-like (a,b) MoS2

(from pure MoS2 powder) and (c) carbon (from pure graphitepowder), generated by irradiation in the solar furnace.




5. Summary

Solar and lamp ablation are powerful techniques toobtain synthetic nanostructures that cannot easily be ob-tained by conventional high-temperature chemical andlaser-ablation methods, for which oven-enhanced reactionenvironment temperatures are limited to ~1100 8C. Con-tinuous solar and lamp irradiation with concentratoroptics have produced nanostructures, the formation ofwhich requires reactor temperatures of at least ~2000–2700 K. A sizable, inhomogeneous, photonically hot an-nealing environment created by thermal radiation fromthe irradiated target appears to facilitate the formationand rapid quenching of metastable nanostructures farfrom equilibrium. The materials studied to date subsumeCs2O, SiO2-x, carbon, WS2, and MoS2, and the range ofnanostructures varies from fullerene-like inorganicclosed-cage and nanotubular structures, to exfoliated gra-phene-like nano-sheets. Exactly why photothermal meth-ods driven by ultra-intense solar and lamp light succeed,where high-temperature chemical and pulsed laser abla-tion procedures do not, remains an unanswered questioncurrently under investigation.

Acknowledgments

Dotan Babai is the recipient of a Howard and LisaWenger graduate scholarship. Reshef Tenne gratefully ac-knowledges (a) the support of European Research Soci-ety project INTIF 226639, (b) the Israel Science Founda-tion, (c) AddNano project 229284 of the FP7 (EU) pro-gram, (d) the Harold Perlman Foundation, and (e) theIrving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging. He holds the Drake Family Chair inNanotechnology and is the director of the Helen andMartin Kimmel Center for Nanoscale Science.

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Received: September 5, 2010Accepted: October 4, 2010




synthesis of inorganic fullerene-like nanostructures by

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