Solid-State Phase Transformation as a Route for the SimultaneousSynthesis and Welding of Single-Crystalline Mg2Si NanowiresYongmin Kang† and Sreeram Vaddiraju*,†,‡

†Department of Materials Science & Engineering, Texas A&M University, College Station, Texas 77843, United States‡Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States

*S Supporting Information

ABSTRACT: A simple, but elegant, strategy for thesimultaneous synthesis and welding of single-crystallineMg2Si nanowires is presented. For the synthesis of Mg2Sinanowires, the solid-state phase transformation of presynthe-sized silicon nanowires was employed. For assembling theMg2Si nanowires via the formation of Mg2Si bridges, the phasetransformation of silica nanoparticle-decorated silicon nano-wires was employed. To circumvent the formation of multipleMg2Si nuclei and hence the phase transformation of single-crystalline Mg2Si nanowires into polycrystalline Mg2Si nanowires,solid-state reaction of silicon nanowire tips with magnesium foils at elevated temperatures of 350−400 °C was employed. In thisprocedure, the supersaturation of the sharp tips of the silicon nanowires with magnesium led to the formation of only one Mg2Sinucleus per nanowire. Growth of these lone nuclei led to the formation of single-crystalline Mg2Si nanowires. Extension of thisprocedure for the phase transformation of silica nanoparticle-coated silicon nanowires led to the formation of Mg2Si nanowireswith Mg2Si bridges between them. The formation of Mg2Si bridges was confirmed by high-resolution transmission electronmicroscopy analysis and further verified by electrical conductivity measurements. Such simultaneous synthesis and assembly ofnanowires will be highly useful in the fabrication of thermoelectric modules from not only Mg2Si but also other metal silicidenanowires.

■ INTRODUCTION

The large-scale deployment of solid-state thermoelectrics offerstwo exciting possibilities: increasing the efficiencies of existingprocesses and systems (e.g., automobiles)1,2 and the generationof renewable energy (e.g., from solar thermal energy).3−5 Thesedevices that convert waste heat into electricity have no movingparts, require minimal maintenance, and are reliable.6 Thesecharacteristic traits make them an attractive option for eitherwaste heat scavenging or renewable energy generation. Thereliability of solid-state thermoelectrics in electricity generationcan be clearly inferred from their use in satellites and spaceprobes.7,8

The first aspect that needs to be addressed for realizing thelarge-scale deployment of thermoelectrics in the energygeneration and usage chain consists of pathways for enhancingtheir efficiencies, beyond that obtained by the state of the art.The performance of thermoelectrics is dependent upon thefigure of merit (zT) of thermoelectric materials used in theirfabrication. The zT of materials, in turn, is dependent upon themagnitudes of electrical and thermal transport through them, inaccordance with the relationship zT = (S2σT)/(κe + κl), wherezT is the figure of merit of the thermoelectric material, S is theSeebeck coefficient, σ is the electrical conductivity, κe is theelectronic contribution to the thermal conductivity, and κl is thelattice contribution to the thermal conductivity. Therefore,enhancing the performance of thermoelectric materials requiresreducing the extent of thermal transport through them, without

simultaneously reducing the extent of electrical transportthrough them. A review of the literature indicates that oneway to accomplish this, while also successfully circumventingthe Wiedemann−Franz law constraint, is through a selectivereduction of the lattice thermal conductivities of materials(κl).

6,9,10 Synthesizing materials in nanowire form is a pathwayfor selectively reducing the κl of materials. Recent theoreticaland experimental studies have clearly proven this possibil-ity.11−13

A second aspect that needs to be addressed for the large-scaledeployment of thermoelectrics in the energy generation andusage chain consists of pathways for lowering their cost. Thiscan be accomplished by designing thermoelectric materialsfrom earth-abundant elements. Furthermore, the nontoxicnature of these elements will be all the more beneficial. Amaterial system that perfectly satisfies these criteria ismagnesium silicide (Mg2Si). Mg2Si is a low-bandgap semi-conductor and has a bandgap of 0.78 eV.14 In the bulk, Mg2Si isknown to be very brittle.15 The zT values of bulk Mg2Si alloyedwith Sn14 and Bi15,16 have been reported to be 0.717 and in therange of 0.5−1.2,14,18 respectively. It was also predicted throughtheoretical modeling by Satyala and Vashaee that reduction ofgrain sizes led to an increase in the zT values of Mg2Si.

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Received: January 15, 2014Revised: April 12, 2014Published: April 14, 2014

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Therefore, it is possible to achieve zT values of ∼1.2 by makingMg2Si in nanowire form with diameters on the order of 5−20nm.19 A primary requirement for accomplishing this task is themass production and assembly of single-crystalline Mg2Sinanowires into bulk thermoelectric devices. The single-crystalline nanowire form of Mg2Si is ideal for this purposebecause it offers the possibility of independently tuningelectrical and thermal transport. While the diameter of thenanowires can be used as a lever to tune lattice thermalconductivity, the single-crystalline nature of the nanowiresoffers the possibility of realizing enhanced electrical con-ductivity.20 Another requirement for realizing enhanced zTvalues in bulk assemblies of nanowires comes during theirassembly. Any assembly strategy developed has to ensure thatthe interfaces between the nanowires are oxide-free. In otherwords, there should be a path for electrical conduction betweenthe nanowires in the assembly.No reports of the synthesis of single-crystalline Mg2Si

nanowires exist in the literature. Similarly, reports that discusstailoring the interfacial chemistries of Mg2Si nanowireassemblies to ensure that the interfaces between the nanowiresare oxide-free also do not exist in the literature. However,synthesis of other metal silicide nanowires has beenaccomplished in the past and reported. These reports includethe phase transformation of silicon nanowires intoMnSi1.75,

21−23 CoSi,24,25 GdSi1.75,26 NiSi,27 NiSi2,

28 and PtSi29

nanowires. In addition to the synthesis of PtSi nanowires, PtSi−Si heterojunctions have also been synthesized and reported.29

Most of these reports relied on the supply of either metal ormetal halides through the vapor phase onto presynthesizedsilicon nanowires for their conversion into metal silicidenanowires. A few others relied on the solid-state diffusion of themetal into silicon nanowires for the synthesis of metal silicidenanowires.27,29

Previously, we have also demonstrated that solid-state phasetransformation of presynthesized silicon nanowires can beemployed for the formation of Mg2Si nanowires.30 Thisprocedure allowed for circumventing the problems typicallyencountered during the direct reaction of silicon andmagnesium, namely, the strong propensity of magnesium tobe oxidized and the large vapor pressure difference betweensilicon and magnesium.30 However, the phase transformation ofsingle-crystalline silicon nanowires by reacting them withmagnesium supplied via the vapor phase led to the formationof polycrystalline Mg2Si nanowires in that case.30 Additionally,the presence of an MgO sheath was observed on top ofnanowires. The formation of MgO is believed to be the resultof the reaction of Mg with the native SiO2 layer present on topof the silicon nanowires (4Mg + SiO2 → 2MgO + Mg2Si). Theassembly of these nanowires using hot isostatic pressing orspark plasma sintering will result in the presence of electricallyinsulating MgO at the interfaces between the nanowires. Thislowers the overall electrical conductivity of the Mg2Si nanowireassemblies.In this context, the aim of this paper is twofold: (i) to tune

the nucleation and growth steps involved in the solid-statephase transformation of silicon nanowires and realize single-crystalline Mg2Si nanowires and (ii) to extend the solid-statephase transformation strategy for the simultaneous synthesisand welding of Mg2Si nanowires presented in this work. Herein,we will demonstrate that solid-state phase transformation (i.e.,reaction of magnesium with silicon or SiO2) can be employedfor the simultaneous formation and welding of single-crystalline

Mg2Si nanowires. More specifically, the welding of Mg2Sinanowires through the formation of Mg2Si bridges betweenthem will be demonstrated. Finally, the effect of this weldingprocess on the electrical properties of Mg2Si nanowireassemblies will be discussed.

■ EXPERIMENTAL SECTIONSilicon nanowires necessary for the synthesis and welding of Mg2Sinanowires were obtained using electroless etching (Figure 1a). Thisprocedure was described in detail previously.30−32 Boron-doped ⟨100⟩-oriented silicon wafers (obtained from University Wafer) wereemployed as the raw materials for the synthesis of silicon nanowires.Following electroless etching, the obtained silicon nanowires wereadditionally etched using a 3 wt % KOH aqueous solution for 2 min toensure that they had sharp tips.33 The diameters of the nanowiresobtained using electroless etching typically ranged from 50 to 100 nm,while the lengths ranged from 4.9 to 5.3 μm. All the phasetransformation experiments were performed using a solid-statereaction. Typically, these experiments involved bringing the as-obtained silicon nanowire arrays or silica nanoparticle-decoratedsilicon nanowire arrays in contact with a polished magnesium foil andheating them to 350−400 °C in a vacuum chamber (Figure S1 of theSupporting Information). Mild manual pressure was employed toensure a good contact between the nanowires and the foil before thestart of the phase transformation experiments. The flexible nature ofthe polished magnesium foil allowed for the formation of a goodcontact. A boron nitride ceramic plate weighing 45 g was placed on topof the silicon nanowires, and the magnesium foil experimental setupaided in ensuring that this contact remained in place throughout thephase transformation process (Figure S1 of the SupportingInformation). These experiments were performed in the presence ofhydrogen and at a pressure of 100 mTorr. The typical duration ofthese experiments was 20−60 min. The lower reaction temperatureensured that the supply of magnesium into silicon nanowires for theformation of Mg2Si occurred only through solid-state diffusion. Noappreciable evaporation of magnesium is expected to occur at thesetemperatures. Therefore, the supply of magnesium via the vapor phaseonto silicon nanowires resulting in Mg2Si nanowire formation is notexpected to occur at these low reaction temperatures. For the synthesisof Mg2Si nanowires, as-obtained silicon nanowires were phasetransformed using this solid-state reaction.

For welding the obtained nanowires and realizing Mg2Si nanowireswelded together with Mg2Si bridges between them, phase trans-formation of silica nanoparticle-decorated silicon nanowires wasemployed. Silica nanoparticle-decorated silicon nanowires necessaryfor this purpose were obtained using the following procedure. Siliconnanowires were first exposed to oxygen plasma for 5 min for theformation of -OH groups on their surfaces. The silicon nanowirearrays were then dipped in a dilute solution of a (3-aminopropyl)-trimethoxysilane-functionalized silica nanoparticle dispersion in waterfor 10 min. Electrostatic attraction between the NH3

+ end groups ontop of the silica nanoparticles and the OH− groups on top of thesilicon nanowires leads to the formation of silica nanoparticle-decorated silicon nanowires.34 The nanowires were then cleaned withexcess deionized (DI) water to remove excess silica nanoparticles. Thesilica nanoparticles used in these experiments had an average diameterof 200 nm, sufficient to bridge the gap (or pitch) between two adjacentsilicon nanowires in the array. These silica nanoparticle-coated siliconnanowires were also phase transformed using the same proceduredescribed above. The obtained Mg2Si nanowires were characterizedusing an array of techniques, including scanning electron microscopy(SEM), transmission electron microscopy (TEM), and X-raydiffractometry (XRD).

For the measurement of the electrical conductivities of thenanowires, they were scraped off the wafers onto pyrolytic BNsubstrates in the form of mats. Silver paste was employed to make fourcontacts with each nanowire mat. The electrical conductivities of thesemats were then measured using the four-point probe method. Thesemeasurements were performed in vacuum at temperatures in the range

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of 325−625 K. The thicknesses of the nanowire mats necessary for thedetermination of the electrical conductivities were measured usingprofilometry and confirmed using electron microscopy measurements.

■ RESULTS AND DISCUSSIONA scanning electron micrograph of an array of as-obtainedsilicon nanowires is depicted in Figure 1a. These silicon

nanowires had diameters in the range of 50−100 nm (Figure1a). As observed in the figure, the nanowires also had sharptips. A micrograph of these nanowires after solid-state phasetransformation into Mg2Si nanowires is presented in Figure 1b.As is clearly evident in the figure, the phase transformationallowed for the retention of the nanowire morphology. Amicrograph of silica nanoparticle-decorated silicon nanowires ispresented in Figure 1c. A micrograph of these nanowires afterphase transformation is presented in Figure 1d. Similar to thecase of silicon nanowires, the nanowire morphology was stillretained after phase transformation (Figure 1d). The sphericalbridges between the nanowires are also clearly observed afterphase transformation (Figure 1d).The XRD pattern of the silica nanoparticle-decorated silicon

nanowires after phase transformation is presented in Figure 2a.Additionally, the XRD pattern of bare silicon nanowires afterphase transformation is presented in Figure 2b. It is clearlyevident from the data that the nanowires are composed of onlyMg2Si after phase transformation (cubic crystal structure with alattice parameter of 0.639 nm).35 This result clearly indicatesthe complete transformation of both silicon nanowires andsilica nanoparticle-decorated silicon nanowires into Mg2Sinanowires.TEM analysis of the phase-transformed nanowires was

performed to determine not only whether the Mg2Si nanowiresformed by the phase transformation process are single-crystalline or polycrystalline but also whether the Mg2Sibridges formed between the nanowires ensure the formationof an oxide-free path (i.e., devoid of MgO) for electricalconduction between them. The results indicated that the phase

transformation of the as-obtained silicon nanowires resulted inthe formation of single-crystalline Mg2Si nanowires (Figure 3a).The analysis indicated the presence of an MgO sheath aroundthe Mg2Si nanowires. Analysis of the diffraction pattern fromthe Mg2Si nanowire shown in Figure 3a indicated that theirgrowth direction was [202] (inset of Figure 3a). TEM analysisof welded nanowires indicated that after phase transformationthey are composed of Mg2Si nanowires welded together viaMg2Si bridges (Figure 3b−d). The phase transformation led tothe formation of a single-crystalline Mg2Si bridge betweenadjacent single-crystalline Mg2Si nanowires, and the bridge isdevoid of the presence of any electrically insulating MgO. Thisresult (Figure 3b−d), in conjunction with the fact that thephase transformation of only silicon nanowires led to theformation of Mg2Si nanowires (Figure 3a), clearly indicatedthat the presence of silica nanoparticles between the siliconnanowires is essential for the formation of Mg2Si bridgesbetween the nanowires.Both the formation of single-crystalline Mg2Si nanowires and

the oxide-free welding of Mg2Si nanowires using phasetransformation can be explained using the following mecha-nisms. Typically, the supply of magnesium through the vaporphase onto silicon nanowires and the subsequent super-saturation of the nanowires with magnesium lead to theformation of multiple Mg2Si nuclei inside each nanowire.Further reaction of silicon with additional magnesium diffusinginto the nanowires leads to the growth of these nuclei and theformation of polycrystalline Mg2Si nanowires.30 However,tuning the experimental conditions to allow for the formationof only one Mg2Si nucleus per nanowire should lead to thegrowth of this nucleus into a single-crystalline Mg2Si nanowire.The experimental procedure employed in this study allows forthis possibility as illustrated in Figure 4a. Heating the siliconnanowire arrays with sharp tips placed in contact with amagnesium foil leads to the formation of a single nucleus at thetip of each nanowire. The formation of a single Mg2Si nucleusinside each nanowire can be confirmed by the fact that atsubstrate temperatures in the range of 350−900 °C, Mg2Sinuclei 3−4 nm in size will be formed. The size of the nuclei wasestimated using nucleation theory as explained below. Thecritical size of the nuclei of the second phase (R*) formedinside a parent phase during phase transformation isdetermined by the equation R* = (2γ)/(ΔGv + ΔGs),

36−38

where ΔGv and ΔGs are the total volume free energy and the

Figure 1. Scanning electron micrographs of (a) as-obtained siliconnanowires and (b) silicon nanowires after solid-state phase trans-formation into Mg2Si nanowires. (c) Micrograph of silica nanoparticle-coated silicon nanowires. (d) Micrograph of Mg2Si nanowires weldedvia the formation of Mg2Si bridges between them. These weldednanowires were obtained by phase transforming silica-coated siliconnanowires depicted in panel c. The phase transformation wasperformed by bringing the nanowires into contact with a magnesiumfoil and heating the resulting setup to elevated temperatures.

Figure 2. (a) XRD pattern of welded Mg2Si nanowires obtained bysolid-state phase transformation of silica nanoparticle-coated siliconnanowires. The complete transformation of the silica nanoparticle-coated silicon nanowires into Mg2Si was observed. (b) XRD pattern ofMg2Si nanowires obtained by solid-state phase transformation ofsilicon nanowires. Similar to the case of welded Mg2Si nanowires, thecomplete transformation of the silicon nanowires into Mg2Sinanowires was observed in this case.

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free energy change resulting from strain, respectively, and γ isthe surface energy.39 The free energy change resulting fromstrain (ΔGs) can be expressed as ΔGs = [2Y(1 + ν)ε2]/(1 −ν),36−38 where Y is the Young’s modulus of the second phase, νis the Poisson ratio of the second phase, and ε is the latticemismatch between the two phases.36 Estimation of the variationin the size of the Mg2Si nuclei formed inside Si nanowires withphase transformation temperature indicated that the size of thenuclei decreases with an increase in the phase transformationtemperature (see Figure S2 of the Supporting Information).The following data aided in this estimation. The crystalstructures of both Mg2Si and Si are cubic (lattice parameters a= 0.639 nm35 and a = 0.543 nm);35 Y(Mg2Si) = 7.6 × 1010 Pa,40

and ν(Mg2Si) = 0.161.41 Therefore, at substrate temperaturesin the range of 350−900 °C, Mg2Si nuclei 3−4 nm in size willbe formed (Figure S2 of the Supporting Information). As thenanowire diameters are in the range of 50−100 nm, thepossibility of forming multiple Mg2Si nuclei inside each

nanowire exists in silicon nanowires with a uniform diameterall along their lengths. However, in tapered nanowires, the sizeof the nanowires at the tip is reduced to sizes on the order ofthe size of the Mg2Si nuclei (see Figure S3 of the SupportingInformation for micrographs of the tips of silicon nanowiresbefore and after KOH etching). This essentially leads to theformation of only one Mg2Si nucleus per nanowire. The growthof this nucleus into a single-crystalline Mg2Si nanowireproceeds via the reaction of silicon of the nanowire withadditional magnesium diffusing through it. The step involved inthe growth of Mg2Si nuclei into single-crystalline Mg2Sinanowires also explains the phenomenon of nanowire welding.As depicted in Figure 3, upon addition of silica nanoparticles

to the silicon nanowires, phase transformation leads to theformation of Mg2Si nanowires with Mg2Si bridges betweenthem. The mechanism underlying this phenomenon ispictorially depicted in Figure 4b. It is well-known that reactionof Mg with silica leads to the formation of Mg2Si and MgO,

Figure 3. HRTEM image and the corresponding SAED pattern of single-crystalline Mg2Si nanowires obtained by solid-state phase transformation ofsilicon nanowires with sharp tips. (b−d) Representative TEM micrographs of welded Mg2Si nanowires obtained by solid-state phase transformationof silica nanoparticle-coated silicon nanowires. As seen in the images, this procedure led to the seamless welding of Mg2Si nanowires. The formationof Mg2Si bridges between the nanowires after welding was clearly observed in the high-resolution TEM image provided in panel d. No MgO phasewas observed at the interface between the welded nanowires.

Figure 4. (a) Schematic representing the steps involved in the solid-state phase transformation of single-crystalline silicon nanowires into single-crystalline Mg2Si nanowires. The sharp tips of the silicon nanowires allow for the formation of a single nucleus within each nanowire when they arebrought into contact with a magnesium foil and heated. The growth of this lone nucleus within each nanowire leads to the formation of single-crystalline Mg2Si nanowires. (b) Schematic representation of the steps involved in the solid-state phase transformation process for obtaining Mg2Sinanowires welded via the formation of Mg2Si bridges. The path for the diffusion of Mg is through the Mg2Si first and then through the silicananoparticles bridging them. As magnesium diffuses through the silica nanoparticle, it reacts with it and forms an Mg2Si bridge between two adjacentMg2Si nanowires.

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according to the reaction 4Mg + SiO2 → Mg2Si + 2MgO.42−44

Although previous studies have indicated that at the microscalethis reaction leads to the formation of alternating layers ofMg2Si and MgO,42,43 TEM analyses of the structures reveal arandom distribution of MgO and Mg2Si within a region atnanoscale.44 The diffusion of magnesium and its reaction withSiO2 are responsible for the formation of Mg2Si and MgO.Previous studies have indicated that the diffusion of magnesiumoccurs preferentially though the Mg2Si phase, and not the MgOphase.43 Therefore, if silicon nanowires bridged together withsilica nanoparticles are brought into contact with a magnesiumfoil and heated (Figure 4b), the formation of a lone Mg2Sinucleus inside and its growth lead to the formation of Mg2Sinanowires (Figure 4a). Any additional magnesium diffusingthrough the Mg2Si nanowires diffuses through the silicananoparticles bridging them and leads to the formation of anMg2Si bridge between the nanowires. Had the reaction ofmagnesium with silica nanoparticles led to the formation of aMg2Si/MgO core/shell nanoparticle, no further diffusion ofmagnesium would have occurred and the reaction would nothave reached completion.Further confirmation of the formation of Mg2Si nanowires

bridged together with Mg2Si bridges comes from electricalconductivity measurements. The electrical conductivities ofboth mats of Mg2Si nanowires and mats of Mg2Si nanowireswelded together via Mg2Si bridges were measured and arepresented in Figure 5. As expected, both the Mg2Si nanowire

mats and welded Mg2Si nanowire mats exhibited semi-conducting behavior. Their conductivities increased exponen-tially with temperature. However, the electrical conductivities ofthe welded Mg2Si nanowire mats were observed to be 2 ordersof magnitude higher than those of the bare Mg2Si nanowiremats that are not welded. The enhanced electrical conductivityin the welded nanowire mats cannot be attributed to densitychanges caused by to the addition of silica nanoparticles. This isbecause only 1 wt % silica nanoparticles were added to thesilicon nanowires to weld them using phase transformation.The higher conductivities of the welded Mg2Si nanowire mat,compared to those of the bare Mg2Si nanowire mats, aretherefore believed to be the result of the absence of insulatingMgO layers at the interfaces between welded Mg2Si nanowires.Still, the electrical conductivities of welded Mg2Si nanowiremats (25 S/m at 625 K) were observed to be lower than thosereported for bulk Mg2Si (850 S/m at 623 K).45,46 This is

believed to be the result of the more porous nature of thewelded nanowire mats employed in this study. It is well-knownthat the porosity of nanomaterial assemblies impacts theirelectrical conductivities.47 An examination of the electricalconductivities of welded Mg2Si nanowire assemblies as afunction of their densities is currently ongoing and will bereported at a later date.

■ CONCLUSIONSIn summary, a simple solid-state phase transformation strategyfor the synthesis and assembly via welding of Mg2Si nanowiresis presented. In this strategy, presynthesized silicon nanowires,obtained by electroless etching, were phase transformed intosingle-crystalline Mg2Si nanowires. To circumvent the for-mation of multiple Mg2Si nuclei within each nanowire and theformation of polycrystalline Mg2Si nanowires, solid-statereaction of sharp silicon nanowires with magnesium foils wasemployed. The supersaturation of the sharp tips of the siliconnanowires with the diffusing magnesium led to the formation ofa single nucleus within each nanowire. The growth of this singlenucleus within each nanowire led to the formation of single-crystalline Mg2Si nanowires. The phase transformation strategywas also extended to the phase transformation of silicananoparticle-bridged silicon nanowires into Mg2Si nanowireswelded together with Mg2Si bridges. It is believed that themagnesium diffusing through the silicon nanowires first andthen the silica nanoparticle bridging the nanowires react withthem and leave an Mg2Si path between the nanowires, therebywelding them together. In the absence of silica nanoparticles,no welding of the nanowires was observed. The formation ofMg2Si bridges between Mg2Si nanowires was further confirmedby the electrical conductivity measurements. Welded nanowireassemblies exhibited conductivities 2 orders of magnitudehigher than those exhibited by nonwelded nanowires. Thisstrategy is simple and can be extended to obtain weldednanowire assemblies of many other metal silicides, in additionto Mg2Si.

■ ASSOCIATED CONTENT

*S Supporting InformationA pictorial representation of the experimental setup, a plotindicating an estimate of the variation of the size of Mg2Sinuclei formed inside silicon nanowires at various temperatures,and high-resolution micrographs of the silicon nanowire tipsbefore and after KOH etching. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: sreeram.vaddiraju@tamu.edu.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge financial support from the NSF/DOEthermoelectrics partnership (CBET 1048702) program. Accessto the materials characterization housed inside the ConnCenter for Renewable Energy at the University of Louisville isgratefully acknowledged. The aid of Dr. Jasek Jasinski of theCenter for the TEM characterization of the nanowires is alsoacknowledged.

Figure 5. Plot comparing the variation of the electrical conductivitieswith temperature of both nonwelded Mg2Si nanowires and weldedMg2Si nanowires. The process of welding enhanced the electricalconductivity of Mg2Si nanowires by approximately 2 orders ofmagnitude.

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