Melting of indium, tin, and zinc nanowires embedded in the poresof anodic aluminum oxide

Yulia Shilyaeva • Sergey Gavrilov • Larisa Matyna

Received: 31 October 2013 / Accepted: 16 July 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract The melting temperature of metal nanostruc-

tures embedded in the matrix is an essential thermody-

namic characteristic and a key parameter of the processes

of their transformation into semiconductor structures. In

this work, great attention is paid to the investigation of the

behavior of one-dimensional metal nanocrystals near the

melting point. For this purpose, the arrays of In, Sn, and Zn

nanowires with different diameters have been electro-

chemically grown in the pores of anodic aluminum oxide

(AAO), which is confirmed by the results of the micros-

copy and the phase X-ray diffraction analysis. The melting

of nanowire arrays with different diameters has been

investigated by means of differential scanning calorimetry

(DSC). Aside from the expected melting temperature

decrease, with decreasing the diameter of nanowires, it has

been established that the melting peaks of nanostructure

arrays have a complex shape that requires detailed elabo-

ration in order to more accurately define the melting tem-

perature. It is shown that the signal waveform while

melting depends on geometric parameters of the structure,

and the peak being mapped onto the DSC curve is the result

of superposition of the melting peaks of nanowires with

several characteristic dimensions. For the arrays of In, Sn,

and Zn nanowires in AAO, there have been defined the

melting temperature values according to the methodology

offered, and there has been presented the dependence of the

melting temperature decrease on the nanowires’ diameter.

Keywords Nanowires � Electrodeposition � Melting �Differential scanning calorimetry � Anodic aluminum oxide

Introduction

Thermodynamic properties of nanostructured materials

have been the subject of intensive investigation for a long

time. The developed surface of nanoparticles contributes

greatly to their properties. Thus, interfaces define the

properties of nanomaterials, which substantially differ from

those of the bulk materials. It is represented brightly

enough by the well-known melting temperature decrease of

small particles while decreasing their sizes [1, 2]. Different

models describing this phenomenon are presented and

analyzed in [3, 4]. Along with the melting temperature

decrease there are well-known works in which the authors

observed the reverse phenomenon, if the particle was

placed in the pores of refractory matrix [5–8], however, in

the system discussed in [8] can be eutectic and peritectic

reactions between the particle and the matrix, that is why

the results can be non-uniform. Nevertheless, the matrix

influence should be taken into account, since one of the

ways of nanocrystal stability increase is enclosing the

thread into a more inert material. For example, nanowires

can be successfully formed by electrochemical deposition

on the basis of porous matrices of anodic aluminum oxide.

The template method of synthesis is a simple, low-tem-

perature, and cost-effective method, and as a result, it is

widely used [9–11]. It is expected that thermal properties

of such nanowires embedded into the matrix will change,

and it will be possible to control these properties [12].

It should be taken into account that the behavior of the

particles, embedded into the pores of refractory matrix,

while melting can have peculiarities connected with the

presence of interfaces, which influence the thermal ranges

of nanomaterials’ stability and require detailed study.

Besides, in the case of real nanosystems, there is always

some distribution of particles according to their size, which

Y. Shilyaeva (&) � S. Gavrilov � L. Matyna

National Research University of Electronic Technology, Bld. 5,

Pas. 4806, Zelenograd, Moscow, Russia

e-mail: shyliaeva@gmail.com

123

J Therm Anal Calorim

DOI 10.1007/s10973-014-4038-2

defines the distribution according to other properties, as

well. Therefore, often the task is to correctly interpret the

measurement results of different characteristics.

The objects of this investigation are the arrays of one-

dimensional nanostructures of low-melting-point metals,

which are advanced materials for different applications,

among which, for example, thermoelectric cooling sys-

tems. Besides, the arrays of low-melting-point nanowires

are of great interest in terms of obtaining semiconductor

one-dimensional structures on their basis, by means of

treatment method at temperatures close to the melting

temperature [13–16]. The obtained arrays of semiconductor

nanowires can find different applications in nanoelectronics

and photonics as a part of devices and instruments [17–19].

In this connection, establishing basic trends in behavior of

metal nanocrystals in the matrices close to the melting

point is the task of an utmost importance.

The melting temperature is one of the thermodynamic

characteristics of nanostructures, which can be directly

measured. Very small number of nano-scale particles dis-

persed in the matrix can be investigated by the method of

differential scanning calorimetry (DSC) having sufficiently

high sensitivity. Besides, the DSC method allows us to

trace the changes in the melting temperature depending on

the sizes of the particles according to Gibbs–Thomson

equation [20, 21].

In this work, there has been investigated the melting of

In, Sn, and Zn nanowires embedded into the pores of

anodic aluminum oxide with different geometric parame-

ters. The study has revealed that the shape of the melting

peak in contrast to the corresponding peak of the bulk

substance often has a complex shape, connected with the

distribution of nanocrystals according to their size. Here,

we illustrate the possibility to estimate the melting tem-

perature based on the comparison of the investigation

results of geometric parameters with the DSC data on the

melting temperature. For that the arrays of In, Sn, and Zn

nanowires have been grown electrochemically in the por-

ous anodic aluminum oxide with different pores’ diame-

ters. The obtained composites have been characterized by

means of scanning electron microscopy (SEM), X-ray

diffraction analysis (XRD), and DSC.

Experimental

Sample preparation

Porous aluminum oxide was formed by a two-step anod-

ization [22] of high-purity aluminum foil using water

solutions of sulphuric, oxalic, and phosphoric acids for

getting pores of different diameters.

After the first anodization step, the oxide layer with low

degree of order was removed, and as a result of this, the

obtained aluminum foil with textured surface was exposed

to the secondary anodization, which led to the formation of

porous oxide with high degree of order.

Electrochemical deposition of In, Sn, and Zn into the

pores of alumina matrix was carried out in the galvano-

static mode at the room temperature in a two-electrode cell.

The current density was 5 mA cm-2, the anode material

was identical to the metal being deposited. The composi-

tion of the solutions used is given in Table 1.

The process was being conducted up to the moment

when on the oxide surface, there appeared a metal film,

which was removed with a soft polishing material. The

composites formed were rinsed with deionized water and

then dried in the nitrogen stream.

Structural characterization

The structure of arrays of In, Sn, and Zn nanocrystals

embedded into the pores of alumina matrix was charac-

terized by means of the phase XRD. The measurements

were conducted by means of multifunctional X-ray device

«X-Ray MiniLab», functioning in the mode of diffrac-

tometer (CuKa radiation, k = 1.54 A, scan H-2H, slits

100–200 lm, 10 s step-1 exposure, Ni filter).

The investigation of the samples’ morphology was car-

ried out by means of scanning electron microscope, Helios

NanoLab 650. Geometric parameters of the composites

were defined by means of statistical analysis of micro-

photographs of the samples’ surface fragment. The number

of pores per the unit of area and their size were calculated

and measured by Axio Vision software. As the defining

value of the pore diameter, there was chosen the prevailing

value of this parameter in the distribution bar chart with the

accuracy ± 2 nm.

Differential scanning calorimetry

The test portions of the obtained samples of 3–5 mg were

investigated by means of differential scanning calorimeter,

Table 1 Composition of solutions for electrochemical deposition

Metal Solution Solution

composition/g L-1

Indium In2(SO4)3 130

Na2SO4�10H2O 10

Tin SnSO4 40

H2SO4 100

Zinc ZnSO4 300

H2SO4 10

Y. Shilyaeva et al.

123

DSC 204 F1 Phoenix (Netzsch). The heating of the

samples was conducted in the pressed aluminum crucibles

at the rate of 10 �C min-1 in the argon atmosphere.

Empty aluminum crucibles were used as a reference. The

sample fragment was cut in the shape of the disk and was

placed on the bottom of the crucible, and then pressed

with the lid with the upturned convex side down, in order

to provide good contact of the material with the bottom of

the crucible. The lid of the crucible was punched before

sealing.

Results and discussion

Figure 1 shows the microphotographs of the cross-section

view of the aluminum oxide with indium nanocrystals and

the empty matrix as a comparison. As it follows from the

results of the microscopy, the degree of fillability of the

pores with the metal is high enough, the deposited metal

homogeneously fills the matrix, and the diameter of the

embedded nanocrystal may be supposed to be equal to the

pore diameter of the anodic oxide.

In Fig. 2, there are given the investigation results of the

crystalline structure of the obtained composites by means

of phase XRD. The fact that there are accurate diffraction

peaks in the spectra of the samples means that in the

samples, there are polycrystallites of the metals being

deposited.

Figure 3 gives the investigation results of the melting

processes of nanowires inside the alumina pores. For cor-

rect interpretation of the results, in the identical conditions,

there were obtained DSC curves for the sample of the

empty alumina matrix and the reference samples, which

represent electrochemically deposited films of In, Sn, and

Zn. In the initial structures of the porous AAO in the range

of temperatures of 25–500 �C, there were not found any

phase transitions. In the DSC curves of the metal films,

there was found the only phase transition at the tempera-

tures corresponding to the melting temperatures of these

metals.

For the AAO samples with metals deposited in the

pores, in the DSC curves in the range of temperatures from

Tm – 50 �C up to Tm ? 50 �C, there was observed the heat

absorption, which we associate with the phase transition in

nanocrystals (melting). As it follows from the data in

Fig. 3, the melting peaks of nanocrystals are shifted to the

area of lower temperatures in relation to the melting peak

of the bulk material, and with decreasing diameter of

nanocrystals, this shift becomes larger. It was noted that the

melting peak of nanocrystals was wider than that of the

bulk material, and it often had a complex shape. In our

Fig. 1 Microphotograph of the

cross-section view of the AAO-

In composite (a) and the empty

AAO matrix (b)

20

Inte

nsity

/a.u

.

30 40

002 101

100

200

101002

112--In

--Sn

--Zn

311

103

50

2θ/°60 70 80

a

b

c

Fig. 2 Difractograms of the composites AAO-In (a), AAO-Sn (b),

and AAO-Zn (c)

Melting of indium, tin, and zinc nanowires

123

opinion, this is due to the distribution of nanowires

according to their size, which, in its turn, follows from the

pore size distribution in alumina matrices.

To define the values of the melting temperature more

accurately, the DSC data were mathematically processed,

the thermal effects being simulated with the help of normal

distribution. Figure 4 shows the typical analysis of the

endothermic melting peak of the AAO-Sn composite, the

microphotograph of which is given in Fig. 5.

As we can see in Fig. 4, the result of such analysis

demonstrates the correlation of the melting temperature

with the nanowire size distribution. In other words, the

resulting DSC signal for the composites being studied

represents a set of overlapped melting peaks of nanowires

with different diameters, and the intensity of these peaks

depends on the number of particles with the corresponding

diameter. So we can conclude that the melting temperature

cannot be accurately defined by the beginning of heat

absorption, expressed in DSC dependence by the deviation

from the linear law.

Thus, most accurately the melting temperature can be

defined after the moment when the initial DSC signal is

represented as a set of some peaks. Then the value of the

melting temperature of the nanowires, having the prevail-

ing value of the diameter which corresponds to the maxi-

mum on the bar chart, will correspond to the temperature of

140

Bulk

25 nm

40 nm

60 nm

90 nm

130 nm

Bulk

25 nm

40 nm

60 nm

90 nm

130 nm

Bulk

25 nm

40 nm

60 nm

90 nm

130 nm

150 160 210 220

Temperature/°C

Hea

t flo

w/m

WE

xo u

p

230 240 400 410 420 430

(a) (b) (c)

Fig. 3 DSC curves of the composites AAO-In (a), AAO-Sn (b), and AAO-Zn (c) with different diameters of nanowires in comparison with DSC

signals of macroscopic metal films

220

–0.3

–0.2

–0.1

0

10

20

225 230Temperature/°C

40 60 80Pore diameter/nm

Num

ber

of p

ores

Hea

t flo

w/m

WE

xo u

p

235

(a)

(b)

Fig. 4 The influence of geometric parameters of the composites on

the shape of the melting peak in the DSC curve

Fig. 5 Microphotograph of the surface of the sample fragment, the

characteristics of which are presented in Fig. 4

Y. Shilyaeva et al.

123

the extrapolated beginning of the peak with the greatest

area. In other words, for the sample the characteristics of

which are presented in Figs. 4 and 5, the melting temper-

ature will be 225 �C with the prevailing diameter of

nanowires equal to 58 nm.

The DSC curves presented in Fig. 3 were analyzed in

accordance with the procedure mentioned above, as a

result, there were defined the values of the melting tem-

perature of In, Sn, and Zn nanowires in the AAO matrix,

which are presented as the dependence of the melting

temperature decrease on the diameter, given in Fig. 6.

Conclusions

In our work, the arrays of In, Sn, and Zn nanowires with

different diameters were electrochemically grown in the

pores of AAO, which was confirmed by the results of the

microscopy and phase XRD. The melting of nanowire

arrays was investigated by means of the DSC method. In

addition to the expected melting temperature decrease with

the diameter decrease, it was established that the melting

peaks of nanowire arrays had a complex shape requiring a

detailed elaboration for more accurate definition of the

melting temperature. By the example of the sample, having

a wide non-uniform distribution of nanowires according to

the diameter, it is shown that the waveform of the signal at

melting depends on the geometric parameters of the

structure, and the peak displayed in the DSC curve is the

result of the superposition of the melting peaks of nano-

wires with several characteristic sizes. For the arrays of In,

Sn, and Zn nanowires in the AAO, there were defined the

values of the melting temperature according to the meth-

odology offered, i.e., according to the extrapolated

beginning of the peak having the greatest area, and there

was presented the dependence of the melting temperature

decrease on the diameter of nanowires.

The obtained data have a great value in understanding

the properties of nanostructured materials, and it is very

important from the practical viewpoint for establishing the

temperature ranges of metal nanowires stability while

planning the synthesis of semiconductor one-dimensional

nanostructures on their basis.

Acknowledgements This work was supported by the Ministry of

education and science of the Russian Federation (Code: 122-GZ-

MFE) and FP7-PEOPLE-2011-IRSES-295273 project ‘‘NANEL’’.

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