Nanoscale

FEATURE ARTICLE

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Metallic glass na

LdNnaTStErcon

Department of Mechanical Engineering, T

79409-3121, USA. E-mail: golden.kumar@tt

Cite this: Nanoscale, 2014, 6, 2027

Received 22nd October 2013Accepted 29th November 2013

DOI: 10.1039/c3nr05645g

www.rsc.org/nanoscale

This journal is © The Royal Society of C

nostructures: fabrication,properties, and applications

Lianci Liu, Molla Hasan and Golden Kumar*

Remarkable progress has beenmade in fabrication and characterization of metal nanostructures because of

their crucial role in energy conversion, nanophotonics, nanoelectronics, and biodiagnostics. Less emphasis

has been placed on the synthesis of nanostructures from metallic alloys, which are better suited than

elemental metals for certain applications such as fuel-cell catalysts. The main challenges in fabrication of

alloy nanostructures are controlling their chemical stoichiometry, crystal structures, and shapes because

of anisotropic nucleation and growth rates. These limitations can be overcome by using metallic glasses

(amorphous metal alloys) which are isotropic and provide additional control handles through their

tunable compositions and degree of crystallinity. Here, we review the recent developments in fabrication

and characterization of metallic glass (MG) nanostructures. The focus is on sub-micron structures

synthesized by unconventional thermoplastic techniques. A concept of self-assembly is introduced for

fashioning functional structures using MG nanostructures as building blocks. The article concludes with a

brief discussion about unique properties and prospective applications of MG nanostructures.

1. Introduction

A demand for controlling materials at sub-micron length scaleis rapidly growing in the elds of electronics,1 optics,2 uidics,3

and biomedical sciences.4,5 Semiconductor and carbon-basedmaterials have been extensively studied for nano-scale appli-cations due to their advanced processing capabilities. A widerange of fabrication techniques such as e-beam lithography,6

nanoimprint lithography (NIL),7 scanning probe lithography,8,9

focused-ion-beam (FIB),10 self-assembly,11 atomic-layer-deposi-tion (ALD),12 and vapor-phase growth of nano-wires/tubes13,14

ianci Liu obtained her BSegree in Materials Science fromanjing University of Aero-autics and Astronautics, Chinand MS degree in Physics fromexas Tech University (TTU).he is currently a PhD student inhe department of Mechanicalngineering at TTU. Heresearch interests are in fabri-ation, self-assembly, andptical properties of metallicanostructures.

exas Tech University, Lubbock, Texas

u.edu

hemistry 2014

have been developed. These efforts have resulted in discovery ofexciting new phenomena and devices such as metal–insulatortransition,15 ballistic conductance,16 quantum dots,17 single-electron transistors,18,19 articial electronic skin,20 and single-cell endoscope.21 The main challenges faced by the existingnanofabrication techniques are complexity, scalability, andlimited applicability to complex materials.

The most efficient processing route to one-dimensionalmetal nanostructures is solution-based reduction of their saltprecursors.14 Nano-wires and nano-tubes from Pt, Pd, Au, andAg have been grown in a controllable manner using thisapproach.22 Electroplating into porous templates is anotherviable method that can produce nanostructures from certainmetals.23 Other techniques such as vapor deposition on

Molla Hasan earned his BSdegree in Mechanical Engi-neering from Khulna Universityof Engineering and Technology(KUET), in Bangladesh. Heobtained his MS in MechanicalEngineering from LamarUniversity. He is currently a PhDstudent in Prof. Golden Kumar'slab at Texas Tech University. Hisresearch interests are in hierar-chical surfaces, tribology, size-effects, and electron microscopy.

Nanoscale, 2014, 6, 2027–2036 | 2027

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patterned substrates are also investigated for fabrication ofnano-patterned metals.24 Besides nano-wires and nano-tubes,nano-scale pores are also highly desirable for application inltration, gas storage, and sensing.25–27 Nano-porous metals aremostly produced by selective chemical etching of one or morecomponents of an alloy.28 The end product is a porous scaffoldmade from an element with higher resistance to etching.29

Therefore, this technique is very effective for synthesis of nano-porous architectures of noble metals (Au, Pt, etc.) by preferentialdissolution of less-noble components such as Cu.30–32

In many applications, alloys are more desirable thanelemental metals because of their superior and tunable prop-erties.33–36 Carefully designed alloy catalysts exhibit higherresistance to CO poisoning in fuel cells.35,37 Alloying iscommonly used for mechanical strengthening of metals.38

Synthesis of alloy nanostructures in controllable sizes andshapes has been a challenging task for solution-based growth,template-based electroplating or vapor deposition, and selectiveetching techniques. Maintaining stoichiometry of multi-element crystalline phases at nano-scale requires a stringentcontrol of growth kinetics and electrochemical reactions.Furthermore, an anisotropic growth along different crystallo-graphic directions dictates the nal shape and properties ofnanostructures.39 These limitations are alleviated in isotropicmetallic glasses (amorphous metal alloys) which can be directlymolded into desirable shapes while retaining their chemicalintegrity.40–42

Metallic glasses (MGs) are a class of alloys which can beprepared in an amorphous state by fast cooling from the liquidstate. These materials are intriguing because of their distinctiveproperties and for their amenability to thermoplastic shaping atall length scales.43–46 Ability to ow under pressure at relativelymoderate temperatures opens a new avenue of plastic pro-cessing methods to metals, allowing access to intricate struc-tures that were challenging with conventional metal formingtechniques. About more than 100 different glass forming alloysbased on Al, Au, Co, Cu, Fe, Pd, Pt, Ti, and Zr have beendeveloped.45,47 These multicomponent alloys exist in a broadrange of compositions that can be further tuned to optimize theproperties. Although the best glass forming compositions are

Golden Kumar obtained his MSdegree in Materials Science fromIndian Institute of Technology(IIT) Kharagpur in India and aPhD degree in Physics fromTechnical University Dresden inGermany. He worked as a post-doc at National Institute forMaterials Science, Tsukuba(Japan) and Yale University. He iscurrently an assistant professorof Mechanical Engineering atTexas Tech University. Professor

Kumar's research focuses on metallic nanostructures, surfaceengineering, and physics of glasses.

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rather narrow, practically usable glassy samples can beproduced over a range of compositions.48,49 It has been reportedthat the properties of MGs can be varied by changing thecomposition,50,51 the structural order,52 and the processingconditions.53,54 A controllable degree of structural or chemicalorder ranging from short-range clusters to long-range crystalscan be induced in MGs by simple heat treatment.55

Access to a broad range of structures and compositions inMGs and their derivatives offers exibility in material selectionand optimization for achieving desirable properties. Thisopportunity has sparked widespread interest in developing MGsfor functional applications.56 A particular emphasis has been onmicro- and nano-scale geometries because of lower materialcost.42 In this feature article, we discuss the recent trends infabrication, characterization, and applications of MGs at sub-micron scale. We conclude with prospects and opportunities forthe future research in this eld. The experimental results pre-sented here are largely from Pt-based (Pt57.5Cu14.7Ni5.3P22.5) andPd-based (Pd43Cu27Ni10P20) MGs. The proposed methods arehowever applicable to other MGs such as Ni-based(Ni60Pd20P17B3), Mg-based (Mg56Cu29.7Ag3.3Y11) and Au-based(Au49Ag5.5Pd2.3Cu26.9Si16.3) which can be thermoplasticallypatterned on the nano-scale. Because of formation of a rigidoxide layer, Zr-based MGs are not suitable for nano-scale ther-moplastic processing under ambient conditions.

2. Fabrication

There are mainly three different ways to fabricate MG nano-structures: direct processing, machining, and thermoplasticmolding. Direct processing requires rapid cooling of nano-sizedsamples from the vapor or liquid state of a metallic alloy. Vapor-phase direct processing can produce thin lms or low-aspect-ratio microstructures in MGs.57 Liquid-phase direct processingrequires lling of nano-sized molds with a metallic liquid fol-lowed by rapid cooling to avoid crystallization. This mustovercome issues such as capillary pressure, trapped bubbles,and high temperature reaction between molten metals andtemplates. Therefore, direct processing techniques can onlyproduce limited shapes and sizes of MG nanostructures understringent experimental conditions. Alternatively, FIB can beused to mill features in a sub-micron range in essentially anymetal or alloy. Fabrication of nanostructures such as posts,gratings, and holes has been demonstrated in a wide range ofMGs using FIB.58 The sequential nature of FIB inhibits itsapplicability for mass production though it will remain anindispensable tool for fashioning nano-scale samples forresearch. In contrast, thermoplastic molding is a highthroughput parallel fabrication method that separates thevolume-limiting step (MG formation) from shaping.59

MGs can be readily prepared in simple geometries such asrods or sheets using metal casting techniques. These shapes areused as a feedstock material for thermoplastic forming (TPF)operations to make complex structures. In TPF, the as-cast MGis heated above the glass transition temperature (Tg), where itsviscous state can conform to the template features (e.g., gearshaped cavities in Fig. 1) under applied pressure. Viscosities in

This journal is © The Royal Society of Chemistry 2014

Fig. 1 Schematic illustration of parallel plate TPF of MGs. Load isapplied to the template and MG assembly, which are heated above Tg.Viscous MG fills the template cavities as demonstrated by an exampleof micro-gears shaped from Pt-based MG.

Fig. 2 SEM images of 100 nmdiameter MG pillars with different aspectratios (AR). The nano-pillars are stable up to AR � 10 and collapse athigher AR values.

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the range of 105 to 109 Pa s can be accessed for a wide range ofMGs in the temperature range of 200–450 �C.60,61 No fast coolingis necessary to retain the glassy structure aer TPF because thecrystallization kinetics are sluggish at typical TPF tempera-tures.59 TPF of MGs has been explored for a variety of reshapingmethods such as micro-patterning,42 nano-patterning,41,62,63

blow molding,64,65 foam-making,66 extrusion,67,68 and rolling.69

Here, we will limit our discussion only to the use of TPF forfabrication of sub-micron MG structures.

2.1 Surface nanostructures

Parallel plate geometry is used in typical thermoplastic moldingexperiments with MGs (Fig. 1). A disk of MG is placed on atemplate with pre-dened cavities. The entire assembly isheated to a temperature above Tg using heated plates. Byapplying load to the plates, cavities ranging from millimeter tofew nanometers can be lled with MGs. Filling kinetics formicroscopic cylindrical molds can be estimated using theHagen–Poiseuille law.70 Other variants have also been devel-oped for lling of irregular shaped micro-molds.71,72 For sub-micron molds, additional pressure drop due to capillary effectsmust be incorporated.62 A universal law has been proposed todescribe the viscous ow of MGs at all scales.42 The parameters

This journal is © The Royal Society of Chemistry 2014

that profoundly affect the outcome of this ow model arepressure, viscosity, time, and wetting. All quantities can beprecisely measured except pressure which changes as a functionof location and time as the disk area increases during molding.This inherent challenge of pressure distribution is manifestedin non-uniform patterns obtained through parallel platemolding. To quantify this effect, we have developed a lubrica-tion model for the calculation of spatial and time variation ofpressure.73 Such pressure values can be used to predict anaspect ratio of nanostructures at different locations in aMG disk.

2.1.1 Vertical nanopatterns. We and others have demon-strated that structures in the range of 10–200 nm can be fabri-cated by template-based TPF under suitable conditions.41,42,63

Fig. 2 shows examples of 100 nm diameter pillars withincreasing aspect ratios (AR) from 5 to 50 obtained by TPF of Pt-based MGs. An increase in AR causes the MG nano-pillars toform bundles and collapse. The collapsing of high-aspect-ratiopillars due to mechanical instability is well understood inpolymeric materials.74 Due to larger elastic moduli of MGscompared to polymers, stable nano-pillars with higher AR canbe designed. Inexpensive porous alumina was used as atemplate, which is commercially available with controllablepore size and spacing. Other templates such as lithographicallypatterned silicon and quartz can also be used. The dimensionsof nanopatterns can be precisely controlled through templateselection and processing parameters (pressure, time, andviscosity). Besides viscosity, other material properties whichinuence the nano-patterning are the wetting properties andhigh temperature oxidation of MGs. The contact angle (q)between MGs and template determines the contribution ofcapillary pressure. For wetting (q < 90�), the capillary pressurefacilitates the ow of MGs into template cavity. In the case ofanti-wetting (q > 90�), the applied load must overcome theopposing capillary pressure to initiate template lling. Foroxidation-prone MGs, the formation of a rigid oxide layer canblock the ow of viscous MGs into the template.73 Therefore,TPF of MGs such as Zr-based should be carried out in an inertenvironment.

2.1.2 Oblique nanopatterns. Orientation of surfacefeatures can profoundly affect the tribological and opticalproperties of materials. Angled features are particularly inter-esting for generating anisotropic adhesion75 and friction.76

Methods proposed for fabrication of slanted patterns requireeither glancing angle vapor deposition77 or templates withslanted features.75 Here, we present a simple approach to alterthe angle of an entire MG nano-pillar or the tip of a nano-pillarin a subsequent processing step (Fig. 3). This technique elimi-nates the need for slanted templates or angled vapor deposi-tion. MG nano-pillars are deformed with a at punch at roomtemperature (Fig. 3a). The bending stress incurred duringdeformation results in formation of inclined nano-pillars atdesired locations. The glassy structure of deformed regions(shear bands) can be restored by short annealing above Tg. Theangle of nano-pillars can be adjusted by controlling the extent ofbending without resorting to new templates. The process can bemodied to bend only the tip of the nano-pillars (Fig. 3b). The

Nanoscale, 2014, 6, 2027–2036 | 2029

Fig. 3 Fabrication of slanted (a) and curved (b) nano-pillars. Forslanted structures, the nano-pillars are deformed with a flat punch atroom temperature (RT). Subsequently, the structure of plasticallydeformed regions can be recovered by short annealing above Tg. Forcurved structures, only tips of nano-pillars are thermoplasticallydeformed with a flat punch heated above Tg. In both cases, the MGbase is not heated from the bottom.

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tips of nano-pillars can be heated and locally deformed by usinga heated (above Tg) punch. The method is self-stabilizingbecause the heat conduction through the MG base cools downnano-pillars, preventing copious deformation. These capabil-ities offer a unique opportunity for systematic study of tribo-logical properties of directionally patterned metallic materials.

2.1.3 Multiscale nanopatterns. Hierarchical morphologyconsisting of multiscale structures has been identied to be acritical aspect for superior control and exibility in modifyingthe surface properties of materials.78–83 We have demonstratedan additive patterning approach based on TPF to combinemultiscale nanostructures on a single MG surface (Fig. 4).Initially, MG is molded into a nano-template with features ofpre-dened shape, size and spacing. The patterned MG is

Fig. 4 Additive patterning approach to generate MG surfaces withnano-scale hierarchy. Patterned MG is created by TPF on a nano-template. The patterned MG is subsequently molded on a secondnano-template with larger features. The processing temperature usedfor the second patterning step is typically lower than that for the firstpatterning. The resulting topography consists of larger nano-featuresdecorated with smaller nano-features. The final morphology dependson the aspect ratio of starting nano-features (a & b) and processingtemperatures used for sequential patterning steps.

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cooled to room temperature and released from the template.Subsequently, the patterned MG is molded into a secondtemplate with larger nano-features. The nal outcome is a MGsurface decorated with multiscale nanostructures stacked inorder of decreasing size (Fig. 4). During each molding step, theprocessing temperature is lowered compared to the precedingstep. This is crucial to retain the shape of nanostructures duringmultiple molding operations. Despite this reduction in pro-cessing temperature, it is difficult to maintain the integrity ofhigh-aspect-ratio nanostructures (Fig. 4a) compared to low-aspect-ratio nanostructures (Fig. 4b). Nevertheless, thesequential patterning can be extended to impose additionallength scales and thus generate ametal surface with unmatchedhierarchy of features in a sub-micron scale. The key require-ments to prevent the smearing of nanostructures duringsubsequent molding are to start with the smallest nano-structures and highest processing temperature.

2.1.4 Smaller nanostructures through size-reduction.Emerging applications of nanodevices, self-assembly, bio-sensors, and scanning probe microscopy rely on imple-mentation of sub-10 nm structures from different materials.While nanofabrication of semiconductors and polymers hassurged into the sub-nanometer regime, metal processing stilllacks a scalable method for sub-10 nm features. By selectingappropriate processing conditions, it is possible to fabricatesub-10 nm MG nanostructures using template-based TPFdescribed here. However, the molding pressure and the costof templates rise rapidly with decreasing feature size. Forexample, the capillary pressure alone increases from 40 to400 MPa when the feature diameter is reduced from 100 to10 nm.62 The majority of nano-templates do not withstandsuch high pressures. To overcome these limitations, we areexploring mechanical distortion of MGs as an alternativeapproach for producing smaller nanostructures. Size-reduc-tion to fabricate smaller features has been investigated for somaterials.84,85

Nano-pillars can be elongated and nano-pores can be shrunkto further reduce their lateral dimensions. In principle, thissize-reduction approach can be realized by different experi-mental set-ups. Fig. 5 illustrates one such set-up designed toreduce the size of pillars and pores. For pillar geometry (Fig. 5a),the MG also lls specically designed roughness in the topheating plate during initial molding (step I). In step II, the topplate is cooled below Tg while maintaining the lower plate attemperature above Tg. Pulling two plates apart causes the MG todraw from the nano-template because of anchoring with the topplate. The size and morphology of the resulting nanostructurescan be altered by adjusting the strain rate (pulling speed), thetemperature mismatch and the deformation mode (Newtonianor non-Newtonian). Our preliminary results demonstrate thefeasibility of such a size-reduction method as shown by the SEMimages in Fig. 5a. By controlling the temperature and pullingspeed, we were able to reduce the diameter of 150 nm rods toabout 40 nm. Higher pulling speed resulted in sharp tips(radius <10 nm) by inducing necking (instability).

Size-reduction for MG pores is schematically depicted inFig. 5b. A disk with large pores is compressed uniaxially to

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Fig. 5 Size-reduction by elongation of nano-pillars (a) andcompression of nano-pores (b). For pillar elongation, the nano-template is secured on the lower heating plate and MG nano-rods areformed using the standard TPF procedure (step I). The upper plate isroughened to increase the adhesion with the MG. In step II, the upperplate is cooled below Tg and the plates are pulled apart. Depending onthe strain rate, MG nano-wires (during slow pulling) or sharp tips (forfast pulling) are formed. For pore size-reduction, MG with nano-poresis compressed between flat plates above Tg. Lateral viscous flowshrinks the pore diameter and increases the pore spacing.

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shrink the pore diameters by lateral ow of MG. The extent ofreduction can be controlled by pressure and MG viscosity. Wehave achieved a pore diameter 10 times smaller than the orig-inal, while maintaining the overall periodicity (not shown here).Both circular and elongated pores can be produced by varyingthe loading geometry. These size-reduction methods offer asimple route to fabricate smaller MG nanostructures of certainshapes without use of expensive templates. Viability ofproposed methods for a controllable nanofabrication of MGnanostructures depends on the precise control of temperature,strain rate, and deformation mode during size-reduction oper-ations. Reproducibility of features produced by size-reduction issatisfactory. However, more deterministic methods are required

Fig. 6 Synthesis of free nano-rods by sonication of patterned MGs. SEMfree MG nano-rods are also shown.

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to improve the uniformity of the shape and size of nal MGnanostructures.

2.2 Free nanostructures

Individual nanostructures are required for characterization ofmany size-dependent properties of materials. Various tech-niques including FIB,86–88 electroplating,89 and TPF54 have beenexplored for harvesting nano-sized testing specimens. Longexposure to high-energy ions incurred during FIB millingaffects the structure and properties of metastable MGs.54,90 Tominimize the effect of ion irradiation, nanostructures can bereleased from pre-patterned MGs by a short FIB process.54

Nevertheless, due to serial processing FIB is not suitable forcopious production of nanostructures needed for character-ization of many properties. Other methods that can yield largequantities of nanostructures are planarization (thermoplasticor mechanical) and sonication of nano-patterned MGs. Weobserved that room temperature sonication can detach MGnanostructures from patterned MGs. Fig. 6 shows examples ofa patterned MG surface before and aer sonication and theresulting free nano-rods. There was no detectable effect ofroom temperature sonication on the structure and propertiesof MG nano-rods. These nano-rods can be picked up with a FIBnano-manipulator for individual mechanical testing54 or canbe collected using a centrifuge technique for thermalanalysis.91

Free MG nanoparticles can also be synthesized by powdermetallurgy routes. The most common approaches for produc-tion of MG nanoparticles are mechanical milling38 and gasatomization.92 The main focus of these studies has been on thepreparation of bulk MGs from compositions which are difficultto vitrify from the liquid state. The MG powders with particlesizes smaller than 100 nm are challenging to obtain bymechanical milling due to agglomeration and cold weldingissues. Therefore, the powder form of MGs has not beenexplored for nanomanufacturing.

2.3 Assembled nanostructures

Self-assembly has emerged as a potential bottom-up fabricationtechnique for complex 3D structures and functional devices.11 Itutilizes interactions between the substrate and nanostructuresor between nanostructures as the driving force for assembly. Avariety of interactions such as capillary,93 electrostatic,94

magnetic,95 and van der Waals96 have been explored. Depending

images show a patterned MG before and after sonication. The resulting

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Fig. 7 Proposed self-assembly of free MG nanostructures andpreliminary results. The MG nano-rods are dispersed in a solvent andallowed to settle on a patterned (topographically or chemically)substrate. In the present case, a partially crystallized MG of the samecomposition as the nano-rods was used as a substrate. The nano-rodspreferentially adhered to the crystalline regions. The number ofadhered nano-rods increased significantly by increasing the timeduration. The mechanism of this selective interaction is not yetunderstood.

Fig. 8 Effect of processing on the tensile fracture morphology of Pt-based MG nano-rods (�100 to 150 nm diameters). Reproduced withpermission.54 Copyright (2013), Nature Publishing Group.

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on the type of nanostructures, assembly can also be directedthrough electric eld,97 magnetic eld,95 conned ow, andpatterned templates.98 Nanostructures from almost every type ofmaterial including metals, polymers, and semiconductors havebeen studied as building blocks for self-assembly. This pro-cessing tool has not yet been applied in MGs largely because ofunavailability of MG nanostructures. The ability to producecontrolled MG nanostructures through TPF opens a new area ofresearch in studying nano-scale interactions and applications ofMG nanostructures. Our preliminary work in this directionsuggests a novel crystallization-induced assembly of MG nano-structures (Fig. 7). The MG nano-rods containing a crystallinesurface layer were dispersed in water using sonication. Thesuspension was poured on a MG substrate made from the samealloy consisting of amorphous (light) and crystalline (dark)regions. The nano-rods preferentially decorated the crystallineregions. There was no difference in topography of amorphousand crystalline regions in the substrate which rules out thesurface roughness as the origin of observed selectivity. Onepossible explanation is the crystal-eld interaction between thecrystalline regions of the substrate and the surface layer ofnano-rods. It should be noted that the randomness in assemblyis directly related to the non-uniform distribution in the sizeand shape of nanostructures.

Although the mechanism of observed assembly of MG nano-rods is not yet understood, these results have two importantimplications. First, they suggest a possibility of self-assemblyin MGs, which can be exploited for building applicationspecic structures such as porous lms. Second, self-assemblymay serve as a high throughput characterization for structuraland chemical heterogeneity in MGs. More studies are beingconducted to understand the interaction between MG nano-structures and substrates with topological and chemicalpatterns.

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3. Properties and applications

The interest in MG nanostructures is twofold: to understand thesize-dependent properties and to utilize MGs in nano-scaleapplications. Size-effects continue to spur scientic curiosity inMG nanostructures as many fundamental questions remainpoorly understood in amorphous materials.86–88,99 For example,MGs lack structural and microstructural length scales, whichare the main origin of size-effects in crystalline metals.100–102

Besides scientic drive, characterization of MG nanostructuresis also crucial for applications because the macroscopicknowledge may not suffice at nano-scale as evidenced by studyof crystalline nanostructures.103 MG nanostructures displaydistinct mechanical, electrochemical, and thermal propertiescompared to the bulk samples because of large surface areasand possible size-effects.

3.1 Mechanical stability

Mechanical response of MG nanostructures has received themost attention due to the quest for a remedy to circumvent theshear-mediated brittle fracture.86,87,99,104–107 It is predicted thatplastic deformation of MGs changes from inhomogeneousbrittle to homogeneous ductile in the sub-micron scale due tocrossover between competing energy terms (elastic and shearband energy).87 Results in both agreement87,88 and disagree-ment99,107 with above hypothesis have been reported. By testingnano-wires subjected to different treatments, we have shownthat the observed discrepancy is related to the processing effectswhich are amplied in small samples.54 Fig. 8 shows the tensilefracture morphology of Pt-based MG nano-wires (diameters:100–150 nm) with different processing history. The as-formednano-wire prepared by TPF shows a typical morphology of abrittle fracture observed in macroscopic samples. However, wefound that the FIB-irradiated nano-wire exhibits ductile fracturemarked by evident necking. This suggests that the brittleness ofmacroscopic MGs can be mitigated in nano-scale samples byappropriate processing. Quantitative stress–strain measure-ments have shown that MGs retain their high strength and

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elasticity at nano-scale.88,108 This presents a superior materialfor nano-scale applications which require mechanical robust-ness. One such application is using MG nanostructures astemplates for nanoimprinting.62 MG nanostructures are moredurable than semiconductor nanostructures currently usedas templates. This can signicantly reduce the cost ofnanoimprinting.

3.2 Thermal stability

The melting temperature shows a strong size-dependence incrystalline metal nanostructures.109,110 Similarly, size-dependentchange in Tg has been reported in polymers.111 No such effectshave been reported for thermal properties of MG nano-structures. A possible cause for the lack of these studies is theinability to fabricate large quantities of MG nanostructuresrequired for thermal characterization. We have been recentlyable to measure the DSC (differential scanning calorimeter)signal for MG nano-rods with diameters in the range of 100–200 nm.91 No difference in Tg compared to the bulk sampleswas observed. However, the crystallization kinetics were accel-erated in nano-rods indicated by a decrease in the onset ofcrystallization temperature. The underlying mechanism is notcompletely understood but the microstructural analysis ofdevitried samples suggests enhanced nucleation in MG nano-rods. Unchanged Tg for the nano-rods is not surprising as MGsare free of any structural features in the range of 100 nm andabove. In polymers, the change in Tg is typically observed whenthe length scale of connement becomes comparable to themolecular chain length.112 The only comparable length scale inMGs is shear-transformation-zones (STZs) which are about 10–100 atoms. Therefore, nano-calorimeters which can measuresuch ultrasmall materials are required to glean dynamics ofSTZs from size-dependent Tg in MGs. The size-dependentmelting temperature of MGs cannot be measured under typicalheating rates because of their devitrication. Such experimentswill yield melting behavior of a crystallized MG forming alloy.To measure the size-dependent melting temperature of MGs intheir glassy state, ultrafast heating rates (>100 K s�1) arerequired to avoid devitrication during heating.61

Fig. 9 Comparison of the electrochemical performance of crystallinePt and Pt-based MG: (a) toward CO oxidation with varying surface areaof the MG catalyst and (b) loss of the electrochemical surface area(ECSA) with the number of CV cycles in nitrogen-purged 0.5 mol L�1

H2SO4 solution. Reproduced with permission.113 Copyright (2011),American Chemical Society.

This journal is © The Royal Society of Chemistry 2014

3.3 Electrochemical activity

Metallic nanostructures play an important role in energyconversion devices such as electrodes for solar cells andbatteries, and catalysts in PEM (Polymer Electrolyte Membrane)fuel cells. The ability to form large surface area structures bysimple molding in conjunction with the chemical diversity isfueling recent interest in MGs for energy applications.32,113–115

Pt-based and Pd-based MGs are of particular interest for testingtheir suitability in fuel-cell catalysts and hydrogen storage. Pt-based MG nanostructures display high catalytic activity towardsCO oxidation (Fig. 9a) and retain more than 90% of theirperformance aer 1000 cycles compared to conventional crys-talline Pt catalysts (Fig. 9b). Due to the effect of alloying, the Pt-based metallic glass catalyst also exhibits higher resistance toCO poisoning, which is a serious concern in fuel-cell catalysts.These preliminary experiments suggest that MGs have greatpotential for electrochemical applications. A proof-of-conceptfor a MG micro-fuel-cell has been demonstrated.115 Largesurface area, mechanical durability, and electrical conductivityof MG nanostructures are attractive attributes for electrodeapplications in batteries and supercapacitors.

3.4 Other unexplored properties

Metals are “high-energy” surfaces, and as a result they wet andcorrode when coming into contact with liquids. Whether theseproperties can be respectively rendered hydrophobic andcorrosion resistant, by surface patterning alone, remains anopen question. Similarly, can the adhesion of metal surfaces becontrolled by applying bioinspired topography similar to poly-mers? MG nanostructures provide an ideal toolbox to studythese questions. Promising results have been obtained in recentstudies on topography controlled wetting of MGs.116,117

Optical and electrical properties of semiconductor, carbon,and crystalline metal nanostructures have been extensivelystudied for applications in photonics, plasmonics, and infraredabsorbers. By comparison, however, the optical and electricalproperties of MG nanostructures are largely unexplored. Thisinformation is needed in its own right as well as for thefundamental understanding of transport mechanisms inamorphous materials. Tribological (adhesion, friction, andwear) behavior of patterned MGs is another uncharted researcharea which is of fundamental and technological importance.

4. Conclusions and perspective

A wide range of chemistry and low-cost nanofabrication of MGsprovide unprecedented control in material properties. Theseadvantages and fundamental interest in understanding of size-dependent properties are driving much interest in nano-scaleMGs. In this article, we review the recent advances in fabricationof MG nanostructures. We propose several variants of TPF forsynthesis of nano-scale surface structures (vertical, angled,multiscale) and free nanostructures. A novel mechanicaldistortion to form even smaller nanostructures is proposed tomitigate the challenges of increasing template cost andmolding pressure with decreasing feature size.

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A bottom-up fabrication technique based on self-assemblyof free nanostructures is introduced for the rst time in MGs.The crystallization-induced assembly demonstrated herepresents just one example of uncontrolled interaction. With abetter understanding of nano-scale interactions and properexperimental design, self-assembly can be used to fabricatedevice-specic MG architectures such as electrodes, catalysts,sensors, etc.

While the fabrication of MG nanostructures has beenstudied in great detail, characterization and applications arestill in infancy. Considering that the research in MG nano-structures is relatively young, understanding and harnessingthe properties of these structures should be viewed as oppor-tunities for a contribution. We believe this review will motivatefuture studies in MGs to focus on characterization of optical,electrical, and tribological properties and advancement of self-assembly of MG nanostructures.

Acknowledgements

This work was supported by the National Science Foundationunder Grant CMMI-1266277.

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