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Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 1037ndash1041 2011

New Synthesis Approach for Low TemperatureBimetallic Nanoparticles Size and Composition

Controlled SnndashCu Nanoparticles

Yun Hwan Jo1 Ji Chan Park2 Jung Up Bang2 Hyunjoon Song2 and Hyuck Mo Lee1lowast1KAIST Department of Materials Science and Engineering Guseong-dong 373-1 Yuseong-gu Daejeon 305-701 South Korea

2KAIST Department of Chemistry Guseong-dong 373-1 Yuseong-gu Daejeon 305-701 South Korea

A various size of SnndashCu nanoparticles were synthesized by using a modified polyol process forlow temperature electronic devices Monodispersive SnndashCu nanoparticles with diameters of 21 nm18 nm and 14 nm were synthesized In addition the eutectic composition shift was also observed innano-sized particles as compared with bulk alloys By controlling the size and eutectic compositiona significant melting temperature depression of 303 C was achieved These melting temperaturedepression approaches will reduce adverse thermal effects in electronic devices and provide thesynthesis guidelines for bimetallic nanoparticles with a low melting temperature

Keywords SnndashCu Nanoparticles Size Control Eutectic Composition Shift Low MeltingTemperature Alloys Modified Polyol Process

1 INTRODUCTION

The high temperature process decreases the reliability ofelectronic assemblies because of the energy consumptionsubstrate warpage and thermal stress Hence the metalnanoparticles with a low melting temperature such asAg Cu Sn and bimetallic compounds were researchedas an electronic device interconnection and flexible dis-play materials1ndash4 Of these alloys the Sn(tin)ndashCu(copper)bimetallic compound is considered as the most promis-ing candidate alloy because of its low melting tempera-ture 227 C the eutectic composition Snndash07Cu (numbersare all in weight percent unless specified otherwise) andrelatively low cost5ndash6 A size-dependent melting pointdepression occurs when the size of a particle reachesthe nanometer scale On account of the melting temper-ature depression of nanometermdashsize particles studies onSn-containing nanoparticles have recently been reportedJiang et al and Hsiao et al used a chemical reduc-tion method to synthesize Snndash35Ag and Snndash35AgndashxCu(x = 02 05 10)7ndash8 Both studies reported the success-ful synthesis of nanoparticles smaller than 20 nm and theobservation of a melting temperature depression Howeverthe nonuniform size of the nanoparticles resulted in severalmelting peaks in differential scanning calorimeter (DSC)analysis and no obvious melting temperature depression

lowastAuthor to whom correspondence should be addressed

was observed To apply metal nanoparticles to electronicdevices we need to ensure that the size distribution ofthe nanoparticles is monodispersive the monodispersivityfacilitates the task of melting and connecting a chip toa substrate at a certain temperature To the best of ourknowledge there has been no report on the SnndashCu bimetal-lic nanoparticle In this paper we synthesized and charac-terized the monodispersive SnndashCu bimetallic nanoparticlesby using a modified polyol process and observed a sig-nificant melting temperature depression by controlling thesize and composition9

2 EXPERIMENTAL DETAILS

21 Preparation of Nanoparticles

A modified polyol process was used to synthesize SnndashCunanoparticles Tin(II) acetate (Sn(C2H3O22 Sigma-Aldrich) and Copper(II) acetylacetonate (Cu(C5H7O22Strem) were used as precursors of the SnndashCu nanoparti-cles The reducing agent surface stabilizer and solventwere NaBH4 (99 Sigma-Aldrich) poly(vinyl pyrroli-done) (PVP) (MW = 55000) and 15 pentanediol (96Sigma-Aldrich) respectively All chemicals were used asreceived without further processing or purificationDifferent sized Snndash07Cu nanoparticles were synthe-

sized by a sequential NaBH4 reduction of metal salts in15 pentanediol All solutions were vacuumed and purged

J Nanosci Nanotechnol 2011 Vol 11 No 2 1533-48802011111037005 doi101166jnn20113052 1037

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New Synthesis Approach for Low Temperature Bimetallic Nanoparticles Jo et al

argon continuously during experiments to prevent fromoxidations This step was followed by various heating tem-peratures under argon in the presence of PVP For thesynthesis of 21 nm and 18 nm Snndash07Cu nanoparticles406 g of PVP (36 mmol 120 equiv) and 0072 g ofSn(C2H3O22 were added to two necks (100 ml) and dis-solved in 45 ml of 15 pentanediol The mixture was thenheated to 200 C and 160 C in a argon atmosphere whilebeing stirred Next NaBH4 (0924 g dissolved in 20 mlof 15 pentanediol) was injected into the solutions After5 min a mixture of 105 mg of Cu(C5H7O22 dissolved in1 ml of 15 pentanediol was added at 200 C and 160 CAfter a further 5 min the resulting solution was cooledThe nanoparticles were subsequently obtained by centrifu-gation at 10000 rpm for 30 min and washed with ethanolseveral times For the 14 nm nanoparticles two hot bathswere used the temperature was 100 C in one bath and200 C in the other A mixture of 406 g of PVP and0072 g of Sn(C2H3O22 dissolved in 45 ml of 15 pentane-diol was heated to 100 C and then injected with 0924 gof NaBH4 dissolved in 20 ml of 15 pentanediol After15 min the mixed solvents that were heated in the 100 Cbath were moved to the other 200 C bath Finally a mix-ture of 105 mg of Cu(C5H7O22 dissolved in 1 ml of15 pentanediol was added for 10 min and the final productwas washed with ethanol by means of centrifugation

22 Characterization

The melting temperature measurements were taken witha DSC (TA Instruments) The heating rate of the DSCwas 3 Cmin from 50 C to 250 C Powder XRD datawere obtained on a Rigaku Dmax-2500 diffractometerwith Cu K radiation and the scan rate was 1min Thesize of nanoparticle was estimated from XRD using Scher-rer equation L = K cos where L is the averageparticle size K is the Scherrer constant related to theshape and index (hkl) of the crystals is the wavelength(015406 nm) of the X-rays is the additional broaden-ing (in radians) and is the Bragg angle respectively10

TEM (EM 912 omega operated at 120 kv) HRTEM (JEOLJEM 2100F operated at 200 kV) and electron diffractionimages were used to determine the structure of the SnndashCunanoparticles Samples for the TEM analysis were pre-pared by dropping an ethanol solution containing SnndashCunanoparticles on copper grids coated with carbon film Forelemental analysis a Perkin-Elmer optima 4300DV induc-tively coupled plasma atomic emission spectrophotometer(ICP-AES) was used Finally XPS (AXIS-NOVA (KratosInc)) was used to investigate the surface oxide layer ofthe SnndashCu nanoparticles

3 RESULTS AND DISCUSSION

The synthesis of SnndashCu nanoparticles was conducted bymeans of a one-pot polyol process in the presence of

PVP During the reaction a coordination bond was formedbetween the Sn2+ ions and the lone-pair electron of theoxygen atom on the carbonyl This bonding of Sn ions tothe PVP reduces the susceptibility of Sn ions to oxida-tion and prevents agglomeration And a mixture of Sn2+

dissolved in 15 pentanediol with PVP turns into Sn nano-crystals as a result of the NaBH4 reducing agent ThenCu(C5H7O22 is injected into the Sn nanocrystals underexcess NaBH4 reducing conditions and SnndashCu nanoparti-cles were synthesized via a galvanic displacement reactionbetween Sn nanoparticles and Cu2+ on the basis of thereduction potentials of Cu2+Cu0 (+034 eV) and Sn2+Sn0

(minus014 eV) as shown in Scheme 1The different sizes of the Snndash07Cu nanoparticles were

synthesized at different temperatures as shown in Figure 1The monodispersive Snndash07Cu nanoparticles with a diame-ter of 21 nm ( le 77 standard deviation) were obtainedat 200 C and those with a diameter of 18 nm ( le 49)were synthesized at 160 C However there was no furtherdecreases in the size of the nanoparticles even at a lowsynthesis temperature For smaller Snndash07Cu nanoparti-cles two hot baths were used the temperature was 100 Cin one bath and 200 C in the other Sn precursors needto be reduced at a low temperature for smaller Sn nano-crystals On the other hands Cu should be injected at ahigh temperature to prevent from Cu oxidation11 Hencethe Sn nanocrystals were reduced by NaBH4 at 100 CA pot containing Sn nanocrystals was then moved to the200 C hot bath and Cu was injected into Sn nano-crystals With this two-step process in a single pot themonodispersive nanoparticles with a diameter of 14 nm( le 49) were synthesized as shown in Figure 1(c)The SAED pattern high resolution transmission electronmicroscopy (HRTEM) and X-ray diffraction (XRD) con-firm the formation of -Sn and SnO2 as shown in theinset of Figures 1(a) 2(a) and 3(a) respectively12ndash14 TheX-ray photoelectron spectroscopy (XPS) analysis was usedto observe the surface oxidation The Sn spectrum showstwo peaks due to the spin-orbit coupling of the 3d statewith a spin-orbit separation of 84 eVone peak at 485 eVis assigned to Sn 3d52 the other peak at 4934 eV isassigned to Sn 3d32

15 For the Snndash07Cu nanoparticlesSn 3d52 region shows two peaks at a binding energy of485 eV indicating pure Sn and 4866 eV indicating SnO2

as shown in Figure 2(b) This indicates that Sn nanopar-ticles were partially oxidized due to the rapid oxidationin air during the separation and sampling processes for

Scheme 1 Schematic diagram for the synthesis of SnndashCunanoparticles

1038 J Nanosci Nanotechnol 11 1037ndash1041 2011

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Jo et al New Synthesis Approach for Low Temperature Bimetallic Nanoparticles

Fig 1 TEM images of Snndash07Cu nanoparticles (a) 21 nm nanoparti-cles ( le 77) synthesized at 200 C (b) 18 nm nanoparticles ( le49 synthesized at 160 C (c) 14 nm nanoparticles ( le 49 synthe-sized using a two-step process at 100 C and 200 C (d) Crystal structureof -Sn with a= 0583 nm and c = 0318 nm which is the same struc-ture shown in the inset of (b) The inset (a) A SAED pattern white andgreen index indicate -Sn and SnO2 respectively (b) A HRTEM imagewhere the interplanar distance of 029 nm indicates the (200) planes ofthe -Sn structure The scale bar represents 20 nm and 1 nm (inset)

analysis however the formation of pure SnndashCu nanoparti-cles might be obtained during the reaction because oxygensource was rigorously eliminated during experiments16ndash17

The thickness of the SnO2 layer does not increase in ambi-ent atmosphere because the oxide layer would functionas a passivating layer to hinder further oxidation15 Thediffraction patterns become broadened as the size of thenanoparticles decreases due to the smallness of the crystal-lites as shown in Figure 3(a)18ndash19 For the Snndash07Cu bulkalloy powder we detected -Sn and Cu6Sn5 peaks How-ever no obvious Cu6Sn5 peak was observed in the XRDpattern of the nanosized particles In addition the peak

480 485 490

Binding energy (eV)

(a)(b)

495 500

Fig 2 (a) HRTEM image of Snndash07Cu nanoparticles The dotted lineindicates the thickness of the oxide layer and the measured thickness isaround 2 nm The scale bar represents 2 nm (b) XPS spectra of Snndash07Cunanoparticles Spectrum assigned to 485 eV and 4866 eV indicates pureSn and SnO2 respectively

(a)

(b)

Fig 3 (a) Powder XRD patterns for different size of Snndash07Cunanoparticles (top) 14 nm nanoparticles (middle) 21 nm nanoparticles(bottom) Snndash07Cu bulk alloy powder24 Rectangular circle and trian-gle indicate -Sn (JCPDS No 04-0673) Cu6Sn5 (JCPDS No 45-1488)and SnO2 (JCPDS No 41-1445) respectively (b) Size dependent melt-ing point depression measured by DSC Each curve represents thermalbehavior of bulk 21 nm 18 nm and 14 nm size Snndash07Cu nanoparticlesfrom bottom to top

intensity of SnO2 was small as compared with that of Snbecause the XRD analysis did not detect the thin surfaceoxidation layer120 The mean size of SnndashCu nanoparti-cles was estimated from the full width at half maximum(FWHM) of the (200) peaks in the XRD patterns shownin Figure 3(a) using Scherrer equations The average sizewas 68 nm 21 nm and 13 nm from bulk to 14 nm sizedSnndashCu nanoparticles and these results were well consis-tent with the size calculated using TEM analysis as canbe seen in Figure 1 Clear and continuous lattice fringeimages were obtained as shown in the inset of Figure 1(b)The measured interplanar distance is 029 nm which cor-responds to the orientation of the (200) planes of -Sn21ndash22

However no lattice fringe was observed in the SnO2 shellindicating that the surface oxide layer is amorphous SnO2

as reported previously141523 The amount of Cu is solittle to form periodic Cu6Sn5 crystal structures in thenanoparticles though the Cu6Sn5 structure is not evidentin HRTEM images To determine the size-dependent melt-ing temperature depression we used DSC analysis as canbe seen in Figure 3(b) The peak melting temperature ofthe Snndash07Cu bulk alloy was 2306 C The peak melt-ing temperatures of the 21 nm 18 nm and 14 nm Snndash07Cu nanoparticles were 2129 C 2079 C and 2052 Crespectively These temperatures are 177 C 227 C and254 C lower than the corresponding temperatures of theSnndash07Cu bulk alloys Another observable feature beside

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New Synthesis Approach for Low Temperature Bimetallic Nanoparticles Jo et al

Table I The ratio of Sn to Cu for SnndashxCu (x = 0 07 21 41 53and 66) from the quantitatively calculated results and ICP results

Calculated ICP resultSn [mg] Cu [mg] ratios [] ratios [] MP (peak) [C]

361 0 100Sn 100Sn 2084 [plusmn08]361 025 Sn07Cu Sn08Cu 2052 [plusmn06]361 076 Sn21Cu Sn25Cu 2038 [plusmn07]361 153 Sn41Cu Sn44Cu 2014 [plusmn09]361 204 Sn53Cu Sn59Cu 2003 [plusmn06]361 255 Sn66Cu Sn73Cu 2421 [plusmn13]

the melting temperature depression was the broadening ofthe melting temperature peak This feature is more evi-dent for smaller nanoparticles than for bulk alloys Kofmanet al reported that the enhanced surface melting causedby the curvature effect and thickness of the liquid layer ismuch greater than that observed in bulk alloys and as aresult induces a broadening of the melting temperature insmaller nanoparticles25

To decrease the melting temperature further we synthe-sized SnndashxCu (x = 0 07 21 41 53 and 66) nanopar-ticles in the same two-step process that was used tosynthesize the 14 nm Snndash07Cu nanoparticles For SnndashCubulk alloys the composition of Snndash07Cu is the eutecticpoint which corresponds to the lowest solid-to-liquid tran-sition point However the melting temperature decreasedto 2003 C for the Snndash53Cu which is 49 C lower

Fig 4 TEM images of SnndashxCu nanoparticles (a) Snndash21Cu with diam-eters of 14 nm ( le 58) (b) Snndash41Cu with diameters of 142 nm ( le5) (c) Snndash53Cu with diameters of 134 nm ( le 72) (d) Snndash66Cuwith diameters of 139 nm ( le 52) Synthesis methods are all thesame except composition Scale bar represents 20 nm

than that of the Snndash07Cu nanoparticles as shown inTable I The melting temperature can be affected by thecomposition and size of bimetallic compounds For thecompositional analysis we conducted inductively cou-pled plasma analysis (ICP) The results confirm that theSnndashxCu nanoparticles were well synthesized as the ini-tially intended composition To see the size effect on themelting temperature more than two hundreds of SnndashxCunanoparticles were selected to calculate the size distribu-tion as shown in Figure 4 The mean diameter is around14 nm and the size deviation is less than 72 indicatingthat the melting temperature depression is not affected bythe size distribution These results confirm that the eutecticcomposition shifted from Snndash07Cu in the bulk phase dia-gram to Snndash53Cu in the 14 nm nanoscale phase diagramThe shift in the eutectic composition can be explained asa result of a substantial increase in the solubility of thenanoparticles as compared to that in the bulk alloy26ndash27

The solubility limit of Cu in Sn in a bulk state is lessthan a 0006 wt under the eutectic temperature thus theCu6Sn5 and -Sn phases are formed for Snndash07Cu bulk

(a)

(b)

Fig 5 (a) Powder XRD patterns for Snndash07Cu and Snndash53Cu nanopar-ticles Rectangular circle and triangle indicate -Sn Cu6Sn5 and SnO2respectively (b) Sn rich side of SnndashCu binary phase diagram Eutec-tic composition shifts from Snndash07Cu in a bulk state phase diagram toSnndash53Cu in a 14 nm nanoscale phase diagram

1040 J Nanosci Nanotechnol 11 1037ndash1041 2011

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Jo et al New Synthesis Approach for Low Temperature Bimetallic Nanoparticles

alloys28 Due to the increase in the solubility limit of Cuin Sn in a nano state no Cu6Sn5 peak was observed forSnndash07Cu as shown in Figure 5(a) For Snndash53Cu nanopar-ticles on the other hand a clear Cu6Sn5 peak was detectedas can be seen in Figure 5(a) This result confirms theincrease in the solubility limit of Cu in Sn in nano-sizedparticles and shows that the composition of Snndash53Cu isthe eutectic point in the 14 nm nanoscale phase diagramas shown in Figure 5(b)

4 SUMMARY

In summary to find the low melting temperature bimetalliccompounds we used a modified polyol process to syn-thesize SnndashCu bimetallic nanoparticles in the presence ofPVP Monodispersive SnndashCu nanoparticles with diametersof 21 nm 18 nm and 14 nm were synthesized and eutecticcomposition shift from Snndash07Cu in a bulk phase diagramto Snndash53Cu in a 14 nm nanoscale phase diagrma wasobserved By controlling the size and composition a sig-nificant melting temperature derpression of up to 303 Cwas achieved These low melting temperature nanoparti-cles will reduce adverse thermal effects thereby increasingthe reliability of electronic devices

Acknowledgments This research was supported byWCU (World Class University) program through theNational Research Foundation of Korea funded by theMinistry of Education Science and Technology (R32-10051) Additional support from Samsung Electro-Mechanics Co LTD is also acknowledged

References and Notes

1 S Jeong K Woo D Kim S Lim J S Kim H Shin Y Xia andJ Moon Adv Funct Mater 18 679 (2008)

2 Q Cui F Gao S Mukherjee and Z Gu Small 5 1246 (2009)3 H-H Lee K-S Chou and K-C Huang Nanotechnology 16 2436

(2005)4 L K Kurihara G M Chow and P E Schoen Nanostructured

Materials 5 607 (1995)

5 W T Chen C E Ho and C R Kao J Mater Res 17 263(2002)

6 D R Frear J W Jang J K Lin and C Zhang JOM 53 28(2001)

7 H Jiang K S Moon F Hua and C P Wong Chem Mat 19 4482(2007)

8 L-Y Hsiao and J-G Duh J Electrochem Soc 152 J105 (2005)9 C Liu X Wu T Klemmer N Shukla X Yang D Weller A G

Roy M Tanase and D Laughlin J Phys Chem B 108 6121(2004)

10 Y Lee J Choi K J Lee N E Scott and D Kim Nanotechnology19 415604 (2008)

11 Z-S Hong Y Cao and J-F Deng Mater Lett 52 34 (2002)12 C Nayral E Viala P Fau F Senocq J-C Jumas A Maisonnat

and B Chaudret Chem Eur J 6 4082 (2000)13 D Grandjean R E Benfield C Nayral A Maisonnat and B Chau-

dret J Phys Chem B 108 8876 (2004)14 B M Leonard and R E Schaak J Am Chem Soc 128 11475

(2006)15 Y Wang J Y Lee and T C Deivaraj J Mater Chem 14 362

(2004)16 J C Park and H Song Chem Mat 19 2706 (2007)17 D Duphil S Bastide and C Levy-Clement J Mater Chem

12 2430 (2002)18 T Ungar Scripta Mater 51 777 (2004)19 M Niederberger M H Bartl and G D Stucky Chem Mater

14 4364 (2002)20 B K Park S Jeong D Kim J Moon S Lim and J S Kim

J Colloid Interf Sci 311 417 (2007)21 N H Chou and R E Schaak J Am Chem Soc 129 7339 (2007)22 H Jiang K-S Moon H Dong F Hua and C P Wong Chem

Phys Lett 429 492 (2006)23 C Nayral T Ould-Ely A Maisonnat B Chaudret P Fau

L Lescouzeres and A Peyre-Lavigne Adv Mater 11 61 (1999)24 Note that Snndash07Cu bulk alloy powder was prepared as follows

After preparing Sn and Cu with purity higher than 9999 both ele-ments were encapsulated under vacuum in quartz tubes and meltedand mixed perfectly Then Snndash07Cu alloy was grounded for thepowder XRD analysis

25 R Kofman P Cheyssac A Aouaj Y Lereah G DeutscherT Ben-David J M Penisson and A Bourret Surf Sci 303 231(1994)

26 W A Jesser R Z Shneck and W W Gile Phys Rev B 69 144121(2004)

27 W A Jesser G J Schiflet G L Allen and J L Crawford MaterRes Innovat 2 211 (1999)

28 L Snugovsky C Cermignani D D Perovic and J W RutterJ Electron Mater 33 1313 (2004)

Received 12 January 2010 Accepted 23 February 2010

J Nanosci Nanotechnol 11 1037ndash1041 2011 1041

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New Synthesis Approach for Low Temperature Bimetallic Nanoparticles Jo et al

argon continuously during experiments to prevent fromoxidations This step was followed by various heating tem-peratures under argon in the presence of PVP For thesynthesis of 21 nm and 18 nm Snndash07Cu nanoparticles406 g of PVP (36 mmol 120 equiv) and 0072 g ofSn(C2H3O22 were added to two necks (100 ml) and dis-solved in 45 ml of 15 pentanediol The mixture was thenheated to 200 C and 160 C in a argon atmosphere whilebeing stirred Next NaBH4 (0924 g dissolved in 20 mlof 15 pentanediol) was injected into the solutions After5 min a mixture of 105 mg of Cu(C5H7O22 dissolved in1 ml of 15 pentanediol was added at 200 C and 160 CAfter a further 5 min the resulting solution was cooledThe nanoparticles were subsequently obtained by centrifu-gation at 10000 rpm for 30 min and washed with ethanolseveral times For the 14 nm nanoparticles two hot bathswere used the temperature was 100 C in one bath and200 C in the other A mixture of 406 g of PVP and0072 g of Sn(C2H3O22 dissolved in 45 ml of 15 pentane-diol was heated to 100 C and then injected with 0924 gof NaBH4 dissolved in 20 ml of 15 pentanediol After15 min the mixed solvents that were heated in the 100 Cbath were moved to the other 200 C bath Finally a mix-ture of 105 mg of Cu(C5H7O22 dissolved in 1 ml of15 pentanediol was added for 10 min and the final productwas washed with ethanol by means of centrifugation

22 Characterization

The melting temperature measurements were taken witha DSC (TA Instruments) The heating rate of the DSCwas 3 Cmin from 50 C to 250 C Powder XRD datawere obtained on a Rigaku Dmax-2500 diffractometerwith Cu K radiation and the scan rate was 1min Thesize of nanoparticle was estimated from XRD using Scher-rer equation L = K cos where L is the averageparticle size K is the Scherrer constant related to theshape and index (hkl) of the crystals is the wavelength(015406 nm) of the X-rays is the additional broaden-ing (in radians) and is the Bragg angle respectively10

TEM (EM 912 omega operated at 120 kv) HRTEM (JEOLJEM 2100F operated at 200 kV) and electron diffractionimages were used to determine the structure of the SnndashCunanoparticles Samples for the TEM analysis were pre-pared by dropping an ethanol solution containing SnndashCunanoparticles on copper grids coated with carbon film Forelemental analysis a Perkin-Elmer optima 4300DV induc-tively coupled plasma atomic emission spectrophotometer(ICP-AES) was used Finally XPS (AXIS-NOVA (KratosInc)) was used to investigate the surface oxide layer ofthe SnndashCu nanoparticles

3 RESULTS AND DISCUSSION

The synthesis of SnndashCu nanoparticles was conducted bymeans of a one-pot polyol process in the presence of

PVP During the reaction a coordination bond was formedbetween the Sn2+ ions and the lone-pair electron of theoxygen atom on the carbonyl This bonding of Sn ions tothe PVP reduces the susceptibility of Sn ions to oxida-tion and prevents agglomeration And a mixture of Sn2+

dissolved in 15 pentanediol with PVP turns into Sn nano-crystals as a result of the NaBH4 reducing agent ThenCu(C5H7O22 is injected into the Sn nanocrystals underexcess NaBH4 reducing conditions and SnndashCu nanoparti-cles were synthesized via a galvanic displacement reactionbetween Sn nanoparticles and Cu2+ on the basis of thereduction potentials of Cu2+Cu0 (+034 eV) and Sn2+Sn0

(minus014 eV) as shown in Scheme 1The different sizes of the Snndash07Cu nanoparticles were

synthesized at different temperatures as shown in Figure 1The monodispersive Snndash07Cu nanoparticles with a diame-ter of 21 nm ( le 77 standard deviation) were obtainedat 200 C and those with a diameter of 18 nm ( le 49)were synthesized at 160 C However there was no furtherdecreases in the size of the nanoparticles even at a lowsynthesis temperature For smaller Snndash07Cu nanoparti-cles two hot baths were used the temperature was 100 Cin one bath and 200 C in the other Sn precursors needto be reduced at a low temperature for smaller Sn nano-crystals On the other hands Cu should be injected at ahigh temperature to prevent from Cu oxidation11 Hencethe Sn nanocrystals were reduced by NaBH4 at 100 CA pot containing Sn nanocrystals was then moved to the200 C hot bath and Cu was injected into Sn nano-crystals With this two-step process in a single pot themonodispersive nanoparticles with a diameter of 14 nm( le 49) were synthesized as shown in Figure 1(c)The SAED pattern high resolution transmission electronmicroscopy (HRTEM) and X-ray diffraction (XRD) con-firm the formation of -Sn and SnO2 as shown in theinset of Figures 1(a) 2(a) and 3(a) respectively12ndash14 TheX-ray photoelectron spectroscopy (XPS) analysis was usedto observe the surface oxidation The Sn spectrum showstwo peaks due to the spin-orbit coupling of the 3d statewith a spin-orbit separation of 84 eVone peak at 485 eVis assigned to Sn 3d52 the other peak at 4934 eV isassigned to Sn 3d32

15 For the Snndash07Cu nanoparticlesSn 3d52 region shows two peaks at a binding energy of485 eV indicating pure Sn and 4866 eV indicating SnO2

as shown in Figure 2(b) This indicates that Sn nanopar-ticles were partially oxidized due to the rapid oxidationin air during the separation and sampling processes for

Scheme 1 Schematic diagram for the synthesis of SnndashCunanoparticles

1038 J Nanosci Nanotechnol 11 1037ndash1041 2011

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Jo et al New Synthesis Approach for Low Temperature Bimetallic Nanoparticles

Fig 1 TEM images of Snndash07Cu nanoparticles (a) 21 nm nanoparti-cles ( le 77) synthesized at 200 C (b) 18 nm nanoparticles ( le49 synthesized at 160 C (c) 14 nm nanoparticles ( le 49 synthe-sized using a two-step process at 100 C and 200 C (d) Crystal structureof -Sn with a= 0583 nm and c = 0318 nm which is the same struc-ture shown in the inset of (b) The inset (a) A SAED pattern white andgreen index indicate -Sn and SnO2 respectively (b) A HRTEM imagewhere the interplanar distance of 029 nm indicates the (200) planes ofthe -Sn structure The scale bar represents 20 nm and 1 nm (inset)

analysis however the formation of pure SnndashCu nanoparti-cles might be obtained during the reaction because oxygensource was rigorously eliminated during experiments16ndash17

The thickness of the SnO2 layer does not increase in ambi-ent atmosphere because the oxide layer would functionas a passivating layer to hinder further oxidation15 Thediffraction patterns become broadened as the size of thenanoparticles decreases due to the smallness of the crystal-lites as shown in Figure 3(a)18ndash19 For the Snndash07Cu bulkalloy powder we detected -Sn and Cu6Sn5 peaks How-ever no obvious Cu6Sn5 peak was observed in the XRDpattern of the nanosized particles In addition the peak

480 485 490

Binding energy (eV)

(a)(b)

495 500

Fig 2 (a) HRTEM image of Snndash07Cu nanoparticles The dotted lineindicates the thickness of the oxide layer and the measured thickness isaround 2 nm The scale bar represents 2 nm (b) XPS spectra of Snndash07Cunanoparticles Spectrum assigned to 485 eV and 4866 eV indicates pureSn and SnO2 respectively

(a)

(b)

Fig 3 (a) Powder XRD patterns for different size of Snndash07Cunanoparticles (top) 14 nm nanoparticles (middle) 21 nm nanoparticles(bottom) Snndash07Cu bulk alloy powder24 Rectangular circle and trian-gle indicate -Sn (JCPDS No 04-0673) Cu6Sn5 (JCPDS No 45-1488)and SnO2 (JCPDS No 41-1445) respectively (b) Size dependent melt-ing point depression measured by DSC Each curve represents thermalbehavior of bulk 21 nm 18 nm and 14 nm size Snndash07Cu nanoparticlesfrom bottom to top

intensity of SnO2 was small as compared with that of Snbecause the XRD analysis did not detect the thin surfaceoxidation layer120 The mean size of SnndashCu nanoparti-cles was estimated from the full width at half maximum(FWHM) of the (200) peaks in the XRD patterns shownin Figure 3(a) using Scherrer equations The average sizewas 68 nm 21 nm and 13 nm from bulk to 14 nm sizedSnndashCu nanoparticles and these results were well consis-tent with the size calculated using TEM analysis as canbe seen in Figure 1 Clear and continuous lattice fringeimages were obtained as shown in the inset of Figure 1(b)The measured interplanar distance is 029 nm which cor-responds to the orientation of the (200) planes of -Sn21ndash22

However no lattice fringe was observed in the SnO2 shellindicating that the surface oxide layer is amorphous SnO2

as reported previously141523 The amount of Cu is solittle to form periodic Cu6Sn5 crystal structures in thenanoparticles though the Cu6Sn5 structure is not evidentin HRTEM images To determine the size-dependent melt-ing temperature depression we used DSC analysis as canbe seen in Figure 3(b) The peak melting temperature ofthe Snndash07Cu bulk alloy was 2306 C The peak melt-ing temperatures of the 21 nm 18 nm and 14 nm Snndash07Cu nanoparticles were 2129 C 2079 C and 2052 Crespectively These temperatures are 177 C 227 C and254 C lower than the corresponding temperatures of theSnndash07Cu bulk alloys Another observable feature beside

J Nanosci Nanotechnol 11 1037ndash1041 2011 1039

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New Synthesis Approach for Low Temperature Bimetallic Nanoparticles Jo et al

Table I The ratio of Sn to Cu for SnndashxCu (x = 0 07 21 41 53and 66) from the quantitatively calculated results and ICP results

Calculated ICP resultSn [mg] Cu [mg] ratios [] ratios [] MP (peak) [C]

361 0 100Sn 100Sn 2084 [plusmn08]361 025 Sn07Cu Sn08Cu 2052 [plusmn06]361 076 Sn21Cu Sn25Cu 2038 [plusmn07]361 153 Sn41Cu Sn44Cu 2014 [plusmn09]361 204 Sn53Cu Sn59Cu 2003 [plusmn06]361 255 Sn66Cu Sn73Cu 2421 [plusmn13]

the melting temperature depression was the broadening ofthe melting temperature peak This feature is more evi-dent for smaller nanoparticles than for bulk alloys Kofmanet al reported that the enhanced surface melting causedby the curvature effect and thickness of the liquid layer ismuch greater than that observed in bulk alloys and as aresult induces a broadening of the melting temperature insmaller nanoparticles25

To decrease the melting temperature further we synthe-sized SnndashxCu (x = 0 07 21 41 53 and 66) nanopar-ticles in the same two-step process that was used tosynthesize the 14 nm Snndash07Cu nanoparticles For SnndashCubulk alloys the composition of Snndash07Cu is the eutecticpoint which corresponds to the lowest solid-to-liquid tran-sition point However the melting temperature decreasedto 2003 C for the Snndash53Cu which is 49 C lower

Fig 4 TEM images of SnndashxCu nanoparticles (a) Snndash21Cu with diam-eters of 14 nm ( le 58) (b) Snndash41Cu with diameters of 142 nm ( le5) (c) Snndash53Cu with diameters of 134 nm ( le 72) (d) Snndash66Cuwith diameters of 139 nm ( le 52) Synthesis methods are all thesame except composition Scale bar represents 20 nm

than that of the Snndash07Cu nanoparticles as shown inTable I The melting temperature can be affected by thecomposition and size of bimetallic compounds For thecompositional analysis we conducted inductively cou-pled plasma analysis (ICP) The results confirm that theSnndashxCu nanoparticles were well synthesized as the ini-tially intended composition To see the size effect on themelting temperature more than two hundreds of SnndashxCunanoparticles were selected to calculate the size distribu-tion as shown in Figure 4 The mean diameter is around14 nm and the size deviation is less than 72 indicatingthat the melting temperature depression is not affected bythe size distribution These results confirm that the eutecticcomposition shifted from Snndash07Cu in the bulk phase dia-gram to Snndash53Cu in the 14 nm nanoscale phase diagramThe shift in the eutectic composition can be explained asa result of a substantial increase in the solubility of thenanoparticles as compared to that in the bulk alloy26ndash27

The solubility limit of Cu in Sn in a bulk state is lessthan a 0006 wt under the eutectic temperature thus theCu6Sn5 and -Sn phases are formed for Snndash07Cu bulk

(a)

(b)

Fig 5 (a) Powder XRD patterns for Snndash07Cu and Snndash53Cu nanopar-ticles Rectangular circle and triangle indicate -Sn Cu6Sn5 and SnO2respectively (b) Sn rich side of SnndashCu binary phase diagram Eutec-tic composition shifts from Snndash07Cu in a bulk state phase diagram toSnndash53Cu in a 14 nm nanoscale phase diagram

1040 J Nanosci Nanotechnol 11 1037ndash1041 2011

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Jo et al New Synthesis Approach for Low Temperature Bimetallic Nanoparticles

alloys28 Due to the increase in the solubility limit of Cuin Sn in a nano state no Cu6Sn5 peak was observed forSnndash07Cu as shown in Figure 5(a) For Snndash53Cu nanopar-ticles on the other hand a clear Cu6Sn5 peak was detectedas can be seen in Figure 5(a) This result confirms theincrease in the solubility limit of Cu in Sn in nano-sizedparticles and shows that the composition of Snndash53Cu isthe eutectic point in the 14 nm nanoscale phase diagramas shown in Figure 5(b)

4 SUMMARY

In summary to find the low melting temperature bimetalliccompounds we used a modified polyol process to syn-thesize SnndashCu bimetallic nanoparticles in the presence ofPVP Monodispersive SnndashCu nanoparticles with diametersof 21 nm 18 nm and 14 nm were synthesized and eutecticcomposition shift from Snndash07Cu in a bulk phase diagramto Snndash53Cu in a 14 nm nanoscale phase diagrma wasobserved By controlling the size and composition a sig-nificant melting temperature derpression of up to 303 Cwas achieved These low melting temperature nanoparti-cles will reduce adverse thermal effects thereby increasingthe reliability of electronic devices

Acknowledgments This research was supported byWCU (World Class University) program through theNational Research Foundation of Korea funded by theMinistry of Education Science and Technology (R32-10051) Additional support from Samsung Electro-Mechanics Co LTD is also acknowledged

References and Notes

1 S Jeong K Woo D Kim S Lim J S Kim H Shin Y Xia andJ Moon Adv Funct Mater 18 679 (2008)

2 Q Cui F Gao S Mukherjee and Z Gu Small 5 1246 (2009)3 H-H Lee K-S Chou and K-C Huang Nanotechnology 16 2436

(2005)4 L K Kurihara G M Chow and P E Schoen Nanostructured

Materials 5 607 (1995)

5 W T Chen C E Ho and C R Kao J Mater Res 17 263(2002)

6 D R Frear J W Jang J K Lin and C Zhang JOM 53 28(2001)

7 H Jiang K S Moon F Hua and C P Wong Chem Mat 19 4482(2007)

8 L-Y Hsiao and J-G Duh J Electrochem Soc 152 J105 (2005)9 C Liu X Wu T Klemmer N Shukla X Yang D Weller A G

Roy M Tanase and D Laughlin J Phys Chem B 108 6121(2004)

10 Y Lee J Choi K J Lee N E Scott and D Kim Nanotechnology19 415604 (2008)

11 Z-S Hong Y Cao and J-F Deng Mater Lett 52 34 (2002)12 C Nayral E Viala P Fau F Senocq J-C Jumas A Maisonnat

and B Chaudret Chem Eur J 6 4082 (2000)13 D Grandjean R E Benfield C Nayral A Maisonnat and B Chau-

dret J Phys Chem B 108 8876 (2004)14 B M Leonard and R E Schaak J Am Chem Soc 128 11475

(2006)15 Y Wang J Y Lee and T C Deivaraj J Mater Chem 14 362

(2004)16 J C Park and H Song Chem Mat 19 2706 (2007)17 D Duphil S Bastide and C Levy-Clement J Mater Chem

12 2430 (2002)18 T Ungar Scripta Mater 51 777 (2004)19 M Niederberger M H Bartl and G D Stucky Chem Mater

14 4364 (2002)20 B K Park S Jeong D Kim J Moon S Lim and J S Kim

J Colloid Interf Sci 311 417 (2007)21 N H Chou and R E Schaak J Am Chem Soc 129 7339 (2007)22 H Jiang K-S Moon H Dong F Hua and C P Wong Chem

Phys Lett 429 492 (2006)23 C Nayral T Ould-Ely A Maisonnat B Chaudret P Fau

L Lescouzeres and A Peyre-Lavigne Adv Mater 11 61 (1999)24 Note that Snndash07Cu bulk alloy powder was prepared as follows

After preparing Sn and Cu with purity higher than 9999 both ele-ments were encapsulated under vacuum in quartz tubes and meltedand mixed perfectly Then Snndash07Cu alloy was grounded for thepowder XRD analysis

25 R Kofman P Cheyssac A Aouaj Y Lereah G DeutscherT Ben-David J M Penisson and A Bourret Surf Sci 303 231(1994)

26 W A Jesser R Z Shneck and W W Gile Phys Rev B 69 144121(2004)

27 W A Jesser G J Schiflet G L Allen and J L Crawford MaterRes Innovat 2 211 (1999)

28 L Snugovsky C Cermignani D D Perovic and J W RutterJ Electron Mater 33 1313 (2004)

Received 12 January 2010 Accepted 23 February 2010

J Nanosci Nanotechnol 11 1037ndash1041 2011 1041

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Jo et al New Synthesis Approach for Low Temperature Bimetallic Nanoparticles

Fig 1 TEM images of Snndash07Cu nanoparticles (a) 21 nm nanoparti-cles ( le 77) synthesized at 200 C (b) 18 nm nanoparticles ( le49 synthesized at 160 C (c) 14 nm nanoparticles ( le 49 synthe-sized using a two-step process at 100 C and 200 C (d) Crystal structureof -Sn with a= 0583 nm and c = 0318 nm which is the same struc-ture shown in the inset of (b) The inset (a) A SAED pattern white andgreen index indicate -Sn and SnO2 respectively (b) A HRTEM imagewhere the interplanar distance of 029 nm indicates the (200) planes ofthe -Sn structure The scale bar represents 20 nm and 1 nm (inset)

analysis however the formation of pure SnndashCu nanoparti-cles might be obtained during the reaction because oxygensource was rigorously eliminated during experiments16ndash17

The thickness of the SnO2 layer does not increase in ambi-ent atmosphere because the oxide layer would functionas a passivating layer to hinder further oxidation15 Thediffraction patterns become broadened as the size of thenanoparticles decreases due to the smallness of the crystal-lites as shown in Figure 3(a)18ndash19 For the Snndash07Cu bulkalloy powder we detected -Sn and Cu6Sn5 peaks How-ever no obvious Cu6Sn5 peak was observed in the XRDpattern of the nanosized particles In addition the peak

480 485 490

Binding energy (eV)

(a)(b)

495 500

Fig 2 (a) HRTEM image of Snndash07Cu nanoparticles The dotted lineindicates the thickness of the oxide layer and the measured thickness isaround 2 nm The scale bar represents 2 nm (b) XPS spectra of Snndash07Cunanoparticles Spectrum assigned to 485 eV and 4866 eV indicates pureSn and SnO2 respectively

(a)

(b)

Fig 3 (a) Powder XRD patterns for different size of Snndash07Cunanoparticles (top) 14 nm nanoparticles (middle) 21 nm nanoparticles(bottom) Snndash07Cu bulk alloy powder24 Rectangular circle and trian-gle indicate -Sn (JCPDS No 04-0673) Cu6Sn5 (JCPDS No 45-1488)and SnO2 (JCPDS No 41-1445) respectively (b) Size dependent melt-ing point depression measured by DSC Each curve represents thermalbehavior of bulk 21 nm 18 nm and 14 nm size Snndash07Cu nanoparticlesfrom bottom to top

intensity of SnO2 was small as compared with that of Snbecause the XRD analysis did not detect the thin surfaceoxidation layer120 The mean size of SnndashCu nanoparti-cles was estimated from the full width at half maximum(FWHM) of the (200) peaks in the XRD patterns shownin Figure 3(a) using Scherrer equations The average sizewas 68 nm 21 nm and 13 nm from bulk to 14 nm sizedSnndashCu nanoparticles and these results were well consis-tent with the size calculated using TEM analysis as canbe seen in Figure 1 Clear and continuous lattice fringeimages were obtained as shown in the inset of Figure 1(b)The measured interplanar distance is 029 nm which cor-responds to the orientation of the (200) planes of -Sn21ndash22

However no lattice fringe was observed in the SnO2 shellindicating that the surface oxide layer is amorphous SnO2

as reported previously141523 The amount of Cu is solittle to form periodic Cu6Sn5 crystal structures in thenanoparticles though the Cu6Sn5 structure is not evidentin HRTEM images To determine the size-dependent melt-ing temperature depression we used DSC analysis as canbe seen in Figure 3(b) The peak melting temperature ofthe Snndash07Cu bulk alloy was 2306 C The peak melt-ing temperatures of the 21 nm 18 nm and 14 nm Snndash07Cu nanoparticles were 2129 C 2079 C and 2052 Crespectively These temperatures are 177 C 227 C and254 C lower than the corresponding temperatures of theSnndash07Cu bulk alloys Another observable feature beside

J Nanosci Nanotechnol 11 1037ndash1041 2011 1039

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New Synthesis Approach for Low Temperature Bimetallic Nanoparticles Jo et al

Table I The ratio of Sn to Cu for SnndashxCu (x = 0 07 21 41 53and 66) from the quantitatively calculated results and ICP results

Calculated ICP resultSn [mg] Cu [mg] ratios [] ratios [] MP (peak) [C]

361 0 100Sn 100Sn 2084 [plusmn08]361 025 Sn07Cu Sn08Cu 2052 [plusmn06]361 076 Sn21Cu Sn25Cu 2038 [plusmn07]361 153 Sn41Cu Sn44Cu 2014 [plusmn09]361 204 Sn53Cu Sn59Cu 2003 [plusmn06]361 255 Sn66Cu Sn73Cu 2421 [plusmn13]

the melting temperature depression was the broadening ofthe melting temperature peak This feature is more evi-dent for smaller nanoparticles than for bulk alloys Kofmanet al reported that the enhanced surface melting causedby the curvature effect and thickness of the liquid layer ismuch greater than that observed in bulk alloys and as aresult induces a broadening of the melting temperature insmaller nanoparticles25

To decrease the melting temperature further we synthe-sized SnndashxCu (x = 0 07 21 41 53 and 66) nanopar-ticles in the same two-step process that was used tosynthesize the 14 nm Snndash07Cu nanoparticles For SnndashCubulk alloys the composition of Snndash07Cu is the eutecticpoint which corresponds to the lowest solid-to-liquid tran-sition point However the melting temperature decreasedto 2003 C for the Snndash53Cu which is 49 C lower

Fig 4 TEM images of SnndashxCu nanoparticles (a) Snndash21Cu with diam-eters of 14 nm ( le 58) (b) Snndash41Cu with diameters of 142 nm ( le5) (c) Snndash53Cu with diameters of 134 nm ( le 72) (d) Snndash66Cuwith diameters of 139 nm ( le 52) Synthesis methods are all thesame except composition Scale bar represents 20 nm

than that of the Snndash07Cu nanoparticles as shown inTable I The melting temperature can be affected by thecomposition and size of bimetallic compounds For thecompositional analysis we conducted inductively cou-pled plasma analysis (ICP) The results confirm that theSnndashxCu nanoparticles were well synthesized as the ini-tially intended composition To see the size effect on themelting temperature more than two hundreds of SnndashxCunanoparticles were selected to calculate the size distribu-tion as shown in Figure 4 The mean diameter is around14 nm and the size deviation is less than 72 indicatingthat the melting temperature depression is not affected bythe size distribution These results confirm that the eutecticcomposition shifted from Snndash07Cu in the bulk phase dia-gram to Snndash53Cu in the 14 nm nanoscale phase diagramThe shift in the eutectic composition can be explained asa result of a substantial increase in the solubility of thenanoparticles as compared to that in the bulk alloy26ndash27

The solubility limit of Cu in Sn in a bulk state is lessthan a 0006 wt under the eutectic temperature thus theCu6Sn5 and -Sn phases are formed for Snndash07Cu bulk

(a)

(b)

Fig 5 (a) Powder XRD patterns for Snndash07Cu and Snndash53Cu nanopar-ticles Rectangular circle and triangle indicate -Sn Cu6Sn5 and SnO2respectively (b) Sn rich side of SnndashCu binary phase diagram Eutec-tic composition shifts from Snndash07Cu in a bulk state phase diagram toSnndash53Cu in a 14 nm nanoscale phase diagram

1040 J Nanosci Nanotechnol 11 1037ndash1041 2011

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Jo et al New Synthesis Approach for Low Temperature Bimetallic Nanoparticles

alloys28 Due to the increase in the solubility limit of Cuin Sn in a nano state no Cu6Sn5 peak was observed forSnndash07Cu as shown in Figure 5(a) For Snndash53Cu nanopar-ticles on the other hand a clear Cu6Sn5 peak was detectedas can be seen in Figure 5(a) This result confirms theincrease in the solubility limit of Cu in Sn in nano-sizedparticles and shows that the composition of Snndash53Cu isthe eutectic point in the 14 nm nanoscale phase diagramas shown in Figure 5(b)

4 SUMMARY

In summary to find the low melting temperature bimetalliccompounds we used a modified polyol process to syn-thesize SnndashCu bimetallic nanoparticles in the presence ofPVP Monodispersive SnndashCu nanoparticles with diametersof 21 nm 18 nm and 14 nm were synthesized and eutecticcomposition shift from Snndash07Cu in a bulk phase diagramto Snndash53Cu in a 14 nm nanoscale phase diagrma wasobserved By controlling the size and composition a sig-nificant melting temperature derpression of up to 303 Cwas achieved These low melting temperature nanoparti-cles will reduce adverse thermal effects thereby increasingthe reliability of electronic devices

Acknowledgments This research was supported byWCU (World Class University) program through theNational Research Foundation of Korea funded by theMinistry of Education Science and Technology (R32-10051) Additional support from Samsung Electro-Mechanics Co LTD is also acknowledged

References and Notes

1 S Jeong K Woo D Kim S Lim J S Kim H Shin Y Xia andJ Moon Adv Funct Mater 18 679 (2008)

2 Q Cui F Gao S Mukherjee and Z Gu Small 5 1246 (2009)3 H-H Lee K-S Chou and K-C Huang Nanotechnology 16 2436

(2005)4 L K Kurihara G M Chow and P E Schoen Nanostructured

Materials 5 607 (1995)

5 W T Chen C E Ho and C R Kao J Mater Res 17 263(2002)

6 D R Frear J W Jang J K Lin and C Zhang JOM 53 28(2001)

7 H Jiang K S Moon F Hua and C P Wong Chem Mat 19 4482(2007)

8 L-Y Hsiao and J-G Duh J Electrochem Soc 152 J105 (2005)9 C Liu X Wu T Klemmer N Shukla X Yang D Weller A G

Roy M Tanase and D Laughlin J Phys Chem B 108 6121(2004)

10 Y Lee J Choi K J Lee N E Scott and D Kim Nanotechnology19 415604 (2008)

11 Z-S Hong Y Cao and J-F Deng Mater Lett 52 34 (2002)12 C Nayral E Viala P Fau F Senocq J-C Jumas A Maisonnat

and B Chaudret Chem Eur J 6 4082 (2000)13 D Grandjean R E Benfield C Nayral A Maisonnat and B Chau-

dret J Phys Chem B 108 8876 (2004)14 B M Leonard and R E Schaak J Am Chem Soc 128 11475

(2006)15 Y Wang J Y Lee and T C Deivaraj J Mater Chem 14 362

(2004)16 J C Park and H Song Chem Mat 19 2706 (2007)17 D Duphil S Bastide and C Levy-Clement J Mater Chem

12 2430 (2002)18 T Ungar Scripta Mater 51 777 (2004)19 M Niederberger M H Bartl and G D Stucky Chem Mater

14 4364 (2002)20 B K Park S Jeong D Kim J Moon S Lim and J S Kim

J Colloid Interf Sci 311 417 (2007)21 N H Chou and R E Schaak J Am Chem Soc 129 7339 (2007)22 H Jiang K-S Moon H Dong F Hua and C P Wong Chem

Phys Lett 429 492 (2006)23 C Nayral T Ould-Ely A Maisonnat B Chaudret P Fau

L Lescouzeres and A Peyre-Lavigne Adv Mater 11 61 (1999)24 Note that Snndash07Cu bulk alloy powder was prepared as follows

After preparing Sn and Cu with purity higher than 9999 both ele-ments were encapsulated under vacuum in quartz tubes and meltedand mixed perfectly Then Snndash07Cu alloy was grounded for thepowder XRD analysis

25 R Kofman P Cheyssac A Aouaj Y Lereah G DeutscherT Ben-David J M Penisson and A Bourret Surf Sci 303 231(1994)

26 W A Jesser R Z Shneck and W W Gile Phys Rev B 69 144121(2004)

27 W A Jesser G J Schiflet G L Allen and J L Crawford MaterRes Innovat 2 211 (1999)

28 L Snugovsky C Cermignani D D Perovic and J W RutterJ Electron Mater 33 1313 (2004)

Received 12 January 2010 Accepted 23 February 2010

J Nanosci Nanotechnol 11 1037ndash1041 2011 1041

Delivered by Ingenta toKorea Advanced Institute of Science amp Technology (KAIST)

IP 14324811822Tue 31 Jul 2012 000305

RESEARCH

ARTIC

LE

New Synthesis Approach for Low Temperature Bimetallic Nanoparticles Jo et al

Table I The ratio of Sn to Cu for SnndashxCu (x = 0 07 21 41 53and 66) from the quantitatively calculated results and ICP results

Calculated ICP resultSn [mg] Cu [mg] ratios [] ratios [] MP (peak) [C]

361 0 100Sn 100Sn 2084 [plusmn08]361 025 Sn07Cu Sn08Cu 2052 [plusmn06]361 076 Sn21Cu Sn25Cu 2038 [plusmn07]361 153 Sn41Cu Sn44Cu 2014 [plusmn09]361 204 Sn53Cu Sn59Cu 2003 [plusmn06]361 255 Sn66Cu Sn73Cu 2421 [plusmn13]

the melting temperature depression was the broadening ofthe melting temperature peak This feature is more evi-dent for smaller nanoparticles than for bulk alloys Kofmanet al reported that the enhanced surface melting causedby the curvature effect and thickness of the liquid layer ismuch greater than that observed in bulk alloys and as aresult induces a broadening of the melting temperature insmaller nanoparticles25

To decrease the melting temperature further we synthe-sized SnndashxCu (x = 0 07 21 41 53 and 66) nanopar-ticles in the same two-step process that was used tosynthesize the 14 nm Snndash07Cu nanoparticles For SnndashCubulk alloys the composition of Snndash07Cu is the eutecticpoint which corresponds to the lowest solid-to-liquid tran-sition point However the melting temperature decreasedto 2003 C for the Snndash53Cu which is 49 C lower

Fig 4 TEM images of SnndashxCu nanoparticles (a) Snndash21Cu with diam-eters of 14 nm ( le 58) (b) Snndash41Cu with diameters of 142 nm ( le5) (c) Snndash53Cu with diameters of 134 nm ( le 72) (d) Snndash66Cuwith diameters of 139 nm ( le 52) Synthesis methods are all thesame except composition Scale bar represents 20 nm

than that of the Snndash07Cu nanoparticles as shown inTable I The melting temperature can be affected by thecomposition and size of bimetallic compounds For thecompositional analysis we conducted inductively cou-pled plasma analysis (ICP) The results confirm that theSnndashxCu nanoparticles were well synthesized as the ini-tially intended composition To see the size effect on themelting temperature more than two hundreds of SnndashxCunanoparticles were selected to calculate the size distribu-tion as shown in Figure 4 The mean diameter is around14 nm and the size deviation is less than 72 indicatingthat the melting temperature depression is not affected bythe size distribution These results confirm that the eutecticcomposition shifted from Snndash07Cu in the bulk phase dia-gram to Snndash53Cu in the 14 nm nanoscale phase diagramThe shift in the eutectic composition can be explained asa result of a substantial increase in the solubility of thenanoparticles as compared to that in the bulk alloy26ndash27

The solubility limit of Cu in Sn in a bulk state is lessthan a 0006 wt under the eutectic temperature thus theCu6Sn5 and -Sn phases are formed for Snndash07Cu bulk

(a)

(b)

Fig 5 (a) Powder XRD patterns for Snndash07Cu and Snndash53Cu nanopar-ticles Rectangular circle and triangle indicate -Sn Cu6Sn5 and SnO2respectively (b) Sn rich side of SnndashCu binary phase diagram Eutec-tic composition shifts from Snndash07Cu in a bulk state phase diagram toSnndash53Cu in a 14 nm nanoscale phase diagram

1040 J Nanosci Nanotechnol 11 1037ndash1041 2011

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ARTIC

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Jo et al New Synthesis Approach for Low Temperature Bimetallic Nanoparticles

alloys28 Due to the increase in the solubility limit of Cuin Sn in a nano state no Cu6Sn5 peak was observed forSnndash07Cu as shown in Figure 5(a) For Snndash53Cu nanopar-ticles on the other hand a clear Cu6Sn5 peak was detectedas can be seen in Figure 5(a) This result confirms theincrease in the solubility limit of Cu in Sn in nano-sizedparticles and shows that the composition of Snndash53Cu isthe eutectic point in the 14 nm nanoscale phase diagramas shown in Figure 5(b)

4 SUMMARY

In summary to find the low melting temperature bimetalliccompounds we used a modified polyol process to syn-thesize SnndashCu bimetallic nanoparticles in the presence ofPVP Monodispersive SnndashCu nanoparticles with diametersof 21 nm 18 nm and 14 nm were synthesized and eutecticcomposition shift from Snndash07Cu in a bulk phase diagramto Snndash53Cu in a 14 nm nanoscale phase diagrma wasobserved By controlling the size and composition a sig-nificant melting temperature derpression of up to 303 Cwas achieved These low melting temperature nanoparti-cles will reduce adverse thermal effects thereby increasingthe reliability of electronic devices

Acknowledgments This research was supported byWCU (World Class University) program through theNational Research Foundation of Korea funded by theMinistry of Education Science and Technology (R32-10051) Additional support from Samsung Electro-Mechanics Co LTD is also acknowledged

References and Notes

1 S Jeong K Woo D Kim S Lim J S Kim H Shin Y Xia andJ Moon Adv Funct Mater 18 679 (2008)

2 Q Cui F Gao S Mukherjee and Z Gu Small 5 1246 (2009)3 H-H Lee K-S Chou and K-C Huang Nanotechnology 16 2436

(2005)4 L K Kurihara G M Chow and P E Schoen Nanostructured

Materials 5 607 (1995)

5 W T Chen C E Ho and C R Kao J Mater Res 17 263(2002)

6 D R Frear J W Jang J K Lin and C Zhang JOM 53 28(2001)

7 H Jiang K S Moon F Hua and C P Wong Chem Mat 19 4482(2007)

8 L-Y Hsiao and J-G Duh J Electrochem Soc 152 J105 (2005)9 C Liu X Wu T Klemmer N Shukla X Yang D Weller A G

Roy M Tanase and D Laughlin J Phys Chem B 108 6121(2004)

10 Y Lee J Choi K J Lee N E Scott and D Kim Nanotechnology19 415604 (2008)

11 Z-S Hong Y Cao and J-F Deng Mater Lett 52 34 (2002)12 C Nayral E Viala P Fau F Senocq J-C Jumas A Maisonnat

and B Chaudret Chem Eur J 6 4082 (2000)13 D Grandjean R E Benfield C Nayral A Maisonnat and B Chau-

dret J Phys Chem B 108 8876 (2004)14 B M Leonard and R E Schaak J Am Chem Soc 128 11475

(2006)15 Y Wang J Y Lee and T C Deivaraj J Mater Chem 14 362

(2004)16 J C Park and H Song Chem Mat 19 2706 (2007)17 D Duphil S Bastide and C Levy-Clement J Mater Chem

12 2430 (2002)18 T Ungar Scripta Mater 51 777 (2004)19 M Niederberger M H Bartl and G D Stucky Chem Mater

14 4364 (2002)20 B K Park S Jeong D Kim J Moon S Lim and J S Kim

J Colloid Interf Sci 311 417 (2007)21 N H Chou and R E Schaak J Am Chem Soc 129 7339 (2007)22 H Jiang K-S Moon H Dong F Hua and C P Wong Chem

Phys Lett 429 492 (2006)23 C Nayral T Ould-Ely A Maisonnat B Chaudret P Fau

L Lescouzeres and A Peyre-Lavigne Adv Mater 11 61 (1999)24 Note that Snndash07Cu bulk alloy powder was prepared as follows

After preparing Sn and Cu with purity higher than 9999 both ele-ments were encapsulated under vacuum in quartz tubes and meltedand mixed perfectly Then Snndash07Cu alloy was grounded for thepowder XRD analysis

25 R Kofman P Cheyssac A Aouaj Y Lereah G DeutscherT Ben-David J M Penisson and A Bourret Surf Sci 303 231(1994)

26 W A Jesser R Z Shneck and W W Gile Phys Rev B 69 144121(2004)

27 W A Jesser G J Schiflet G L Allen and J L Crawford MaterRes Innovat 2 211 (1999)

28 L Snugovsky C Cermignani D D Perovic and J W RutterJ Electron Mater 33 1313 (2004)

Received 12 January 2010 Accepted 23 February 2010

J Nanosci Nanotechnol 11 1037ndash1041 2011 1041

Delivered by Ingenta toKorea Advanced Institute of Science amp Technology (KAIST)

IP 14324811822Tue 31 Jul 2012 000305

RESEARCH

ARTIC

LE

Jo et al New Synthesis Approach for Low Temperature Bimetallic Nanoparticles

alloys28 Due to the increase in the solubility limit of Cuin Sn in a nano state no Cu6Sn5 peak was observed forSnndash07Cu as shown in Figure 5(a) For Snndash53Cu nanopar-ticles on the other hand a clear Cu6Sn5 peak was detectedas can be seen in Figure 5(a) This result confirms theincrease in the solubility limit of Cu in Sn in nano-sizedparticles and shows that the composition of Snndash53Cu isthe eutectic point in the 14 nm nanoscale phase diagramas shown in Figure 5(b)

4 SUMMARY

In summary to find the low melting temperature bimetalliccompounds we used a modified polyol process to syn-thesize SnndashCu bimetallic nanoparticles in the presence ofPVP Monodispersive SnndashCu nanoparticles with diametersof 21 nm 18 nm and 14 nm were synthesized and eutecticcomposition shift from Snndash07Cu in a bulk phase diagramto Snndash53Cu in a 14 nm nanoscale phase diagrma wasobserved By controlling the size and composition a sig-nificant melting temperature derpression of up to 303 Cwas achieved These low melting temperature nanoparti-cles will reduce adverse thermal effects thereby increasingthe reliability of electronic devices

Acknowledgments This research was supported byWCU (World Class University) program through theNational Research Foundation of Korea funded by theMinistry of Education Science and Technology (R32-10051) Additional support from Samsung Electro-Mechanics Co LTD is also acknowledged

References and Notes

1 S Jeong K Woo D Kim S Lim J S Kim H Shin Y Xia andJ Moon Adv Funct Mater 18 679 (2008)

2 Q Cui F Gao S Mukherjee and Z Gu Small 5 1246 (2009)3 H-H Lee K-S Chou and K-C Huang Nanotechnology 16 2436

(2005)4 L K Kurihara G M Chow and P E Schoen Nanostructured

Materials 5 607 (1995)

5 W T Chen C E Ho and C R Kao J Mater Res 17 263(2002)

6 D R Frear J W Jang J K Lin and C Zhang JOM 53 28(2001)

7 H Jiang K S Moon F Hua and C P Wong Chem Mat 19 4482(2007)

8 L-Y Hsiao and J-G Duh J Electrochem Soc 152 J105 (2005)9 C Liu X Wu T Klemmer N Shukla X Yang D Weller A G

Roy M Tanase and D Laughlin J Phys Chem B 108 6121(2004)

10 Y Lee J Choi K J Lee N E Scott and D Kim Nanotechnology19 415604 (2008)

11 Z-S Hong Y Cao and J-F Deng Mater Lett 52 34 (2002)12 C Nayral E Viala P Fau F Senocq J-C Jumas A Maisonnat

and B Chaudret Chem Eur J 6 4082 (2000)13 D Grandjean R E Benfield C Nayral A Maisonnat and B Chau-

dret J Phys Chem B 108 8876 (2004)14 B M Leonard and R E Schaak J Am Chem Soc 128 11475

(2006)15 Y Wang J Y Lee and T C Deivaraj J Mater Chem 14 362

(2004)16 J C Park and H Song Chem Mat 19 2706 (2007)17 D Duphil S Bastide and C Levy-Clement J Mater Chem

12 2430 (2002)18 T Ungar Scripta Mater 51 777 (2004)19 M Niederberger M H Bartl and G D Stucky Chem Mater

14 4364 (2002)20 B K Park S Jeong D Kim J Moon S Lim and J S Kim

J Colloid Interf Sci 311 417 (2007)21 N H Chou and R E Schaak J Am Chem Soc 129 7339 (2007)22 H Jiang K-S Moon H Dong F Hua and C P Wong Chem

Phys Lett 429 492 (2006)23 C Nayral T Ould-Ely A Maisonnat B Chaudret P Fau

L Lescouzeres and A Peyre-Lavigne Adv Mater 11 61 (1999)24 Note that Snndash07Cu bulk alloy powder was prepared as follows

After preparing Sn and Cu with purity higher than 9999 both ele-ments were encapsulated under vacuum in quartz tubes and meltedand mixed perfectly Then Snndash07Cu alloy was grounded for thepowder XRD analysis

25 R Kofman P Cheyssac A Aouaj Y Lereah G DeutscherT Ben-David J M Penisson and A Bourret Surf Sci 303 231(1994)

26 W A Jesser R Z Shneck and W W Gile Phys Rev B 69 144121(2004)

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28 L Snugovsky C Cermignani D D Perovic and J W RutterJ Electron Mater 33 1313 (2004)

Received 12 January 2010 Accepted 23 February 2010

J Nanosci Nanotechnol 11 1037ndash1041 2011 1041