ORIGINAL RESEARCH

Synthesis, characterizations, and optical properties of copperselenide quantum dots

Pushpendra Kumar • Kedar Singh

Received: 8 September 2010 / Accepted: 13 November 2010 / Published online: 30 November 2010

� Springer Science+Business Media, LLC 2010

Abstract We demonstrate the synthesis of copper sele-

nide quantum dots (QDs) by element directed, inexpensive,

straight forward wet chemical method which is free from

any surfactant or template. Copper selenide QDs have been

synthesized by elemental copper and selenium in the

presence of ethylene glycol, hydrazine hydrate, and a

defined amount of water at 70 �C within 8 h. The product

is in strong quantum confinement regime, phase analysis,

purity and morphology of the product has been well studied

by X-ray diffraction (XRD), UV–Visible spectroscopy

(UV–Vis), Photo-luminescent spectroscopy (PL), Fourier

transform infrared spectroscopy (FTIR), Transmission

electron microscopy (TEM), High resolution transmission

electron microscopy (HRTEM), and by Atomic force

microscopy (AFM) techniques. The absorption and pho-

toluminescence studies display large ‘‘blue shift’’. TEM

and HRTEM analyses revealed that the QDs diameters are

in the range 2–5 nm. Due to the quantum confinement

effect copper selenide QDs could be potential building

blocks to construct functional devices and solar cell.

The possible mechanism is also discussed.

Keywords Atomic force microscopy � Blue shift �FTIR spectroscopy � Semiconductor nanoparticles �Transmission electron microscopy �UV–Vis spectroscopy

Introduction

Nanostructured materials exhibiting size quantization

effect are of enforced allegation owing to their peerless

dimension-dependent properties and promising applica-

tions as building blocks in electronics, optoelectronics,

sensors and actuators and in bioimaging. Semiconductor

quantum dots (QDs) one of the most important class of

nanostructured materials are nanometer-size fragments of

the corresponding bulk crystals with nearly equal size

distribution and circular in shape with good luminescence

properties, which have been active targets for chemist,

material scientist, and nanotechnologist in recent years due

to their size-dependent properties, flexible processing, and

easier synthetic protocols. Chemically synthesized semi-

conductor nanostructures and their assembly with con-

trolled size are of key importance in material chemistry,

material science, and nanotechnology because of their

importance in optoelectronics, photo-voltaic devices, and

solar cells due to their unique dimension-dependent and

exceptional properties, which differ from those of their

bulk counterpart. Recently wide ranges of techniques have

been developed to synthesize metal chalcogenide with

control of their microstructure and particle size. Copper

chalcogenides have been studied to a lesser extent to its

closest of kin chalcogenides nanoparticulate (especially

CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, PbS, PbTe, and PbSe)

[1–4]. Copper chalcogenides have large number of appli-

cations in various devices such as in solar cells, super ionic

conductors, photo detectors, photo thermal conversion,

electro conductive electrodes, microwave shielding,

coating, thermoelectric cooling, optical filter, and as a

optical recording material [5–12]. Few pioneering

reports have described synthetic route for copper selenide

nanostructures preparation, focusing on their structural

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11224-010-9698-3) contains supplementarymaterial, which is available to authorized users.

P. Kumar � K. Singh (&)

Department of Physics, Faculty of Science,

Banaras Hindu University, Varanasi 221005, India

e-mail: kedarbhp@rediffmail.com

123

Struct Chem (2011) 22:103–110

DOI 10.1007/s11224-010-9698-3

characterization and their photoluminescence properties

[13–18]. Copper selenide may be found in many phases

and structural forms: different stoichiometries such as

CuSe, Cu2Se, Cu2Sex, CuSe2, a-Cu2Se, Cu3Se2, Cu5Se4,

Cu7Se4, etc. as well with non-stoichiometric form such as

Cu2-xSe and can be constructed into several crystallo-

graphic forms (monoclinic, cubic, tetragonal, hexagonal,

etc.) [19]. Special constitutions and properties of these

compositions make copper selenide an ideal candidate for

scientific research. Therefore, considerable progress on the

study of copper selenide has been made in recent years. It

has been reported that thermal stability and band gaps of

copper selenide vary depending on their stoichiometries or

phases [20, 21]. The composition and the crystal structure

of the final products are usually dependent on the prepa-

ration method [22–25]. However, the preparation methods

and size or shape control are less flexible than other

material selenides such as ZnSe and CdSe, and the studies

relating to their optical properties are limited. Therefore, to

develop new methods for preparing high quality copper

selenide nanomaterials and to achieve control of their size

or shape are very necessary. Copper selenide, due to its

stoichiometric variation, is reported to posses a direct band

gap of 2.0–2.2 eV, as well as to an indirect band gap of

1.4 eV (Cu2-xSe for x = 0.2) [26–38]. Obtaining novel

nanomaterials with controllable size or shape under mild

conditions and with safe precursor at lower temperature

with a relatively quicker process is an issue that has

engaged many researchers. Copper selenide, a p-type

semiconductor having excellent electrical and optical

properties, is suitable for photovoltaic application.

Recently, we have developed a general aqueous solution-

phase strategy to grow nanostructured metal chalcogenides

through electromagnetic stirring refluxing technique under

mild conditions which is low cost and effective method

[39–42]. As a common reducing agent for the synthesis of

chalcogenides, N2H4�H2O is hazardous to the environment

[43, 44].

Herein, the diffusion of metallic copper into the sele-

nium in dispersed phase is reported with simple aqueous

solution method leaving behind contaminants. Surface

energy and interparticle affinity for size-dependent diffu-

sion have been shown elegantly leading to the evolution of

copper selenide in the nanometer-size regime by inexpen-

sive, straight forward, and element directed wet chemical

method which is free from any capping agent, chelating

agent, surfactant, or template. Simple precursors elemental

selenium and copper have been utilized for the preparation

of final product. The results showed that copper selenide

QDs can be obtained, and the formation mechanism is also

discussed. To the best of our knowledge, this is the first

report on copper selenide QDs by element directed aqueous

solution method by mixing preformed selenium and copper

in definite proportions.

Experimental details

Synthesis of high quality copper selenide QDs

In a typical synthesis of copper selenide QDs, highly pure

elemental copper (99.999%) and selenium (99.999%)

powders, purchased from Alfa, were used without further

purification. Ethylene glycol and hydrazine hydrate of

analytical grade, purchased from Merck, Germany were

used as received. In synthesis elemental copper (0.80 g) and

selenium (0.40 g) were taken with deionized water, ethyl-

ene glycol, and hydrazine hydrate in the volume ratio of

10:3:1.5, respectively, in a 200 mL capacity conical flask.

Then the solution was refluxed under vigorous stirring at

70 �C for 8 h. Finally, the black precipitates were collected

and washed with anhydrous ethanol and hot distilled water

for several times, then dried in vacuum at 50 �C for 6 h.

Characterizations

The X-ray diffraction (XRD) pattern of as-synthesized

freshly dried powder was recorded by Rigaku Rotoflux

diffractometer (operating at 40 kV, 100 mA) with Cu Karadiation. UV–Visible spectrum was recorded at room

temperature by UV–Vis160 Spectrophotometer (Shima-

dzu, Japan) in the spectral range between 350 and 600 nm

using a spectral bandwidth of 1 nm. Global photolumi-

nescence spectra of copper selenide were recorded by a

computer controlled rationing luminescence spectrometer

(LS55-Perkin Elmer Instruments, UK) with accuracy =

±1.0 nm and reproducibility = ±0.5 nm. A tunable 2 kW

pulse \10 ls from a xenon discharge lamp was used as

the excitation source. A gated photomultiplier tube was

used as a detector. Before the photo-luminescent spec-

troscopy (PL) experiments, the signal to noise ratio was

adjusted to 500:1 using the Raman band of water with

excitation in the range 250 nm depending on particle size.

Transmission electron microscopy (TEM) and high reso-

lution transmission electron microscopy (HRTEM) inves-

tigations were carried out using Tecnai 20G2-TEM,

typical e-beam voltage employed 200 kV. Atomic force

microscopy (AFM) measurements were conducted using

Molecular Imaging, USA make AFM equipment in non-

contact, acoustic AC (AAC) mode. Fourier transform

infrared spectroscopy (FTIR) measurement was carried

out on Varian 3100 FTIR (USA) at room temperature with

as-prepared sample milled in KBr.

104 Struct Chem (2011) 22:103–110

123

Results and discussion

Structural characterizations

Powder XRD pattern of the same copper selenide QDS is

shown in Fig. 1. The XRD pattern of the copper selenide

QDs had characteristic feature corresponding to (541),

(461), (211), (410), (060), (371), (030), (211), (090), (322),

(701), (250), (750), (311), (650), (511), (071), (920), (282),

(400), and (940) planes, which is in a very good accordance

with Cu2Sex (P) structure (Joint Committee on Powder

Diffraction Standers) JCPDS, card no. 47-1448, of Cu2Sex

(a = 13.80 A, b = 20.39 A, and c = 3.923 A). As expec-

ted, the XRD peaks of the sample were considerably

broadened compared to bulk material due to small size of

the QDs. The sizes of the QDs were estimated using

Scherrer formula:

A ¼ 0:94k=b cosh ð1Þ

where A is the average crystallite size, b is the full-widths-

at-half-maximum (FWHM) of the diffraction peak, k(1.5418 A) is the wavelength of X-ray radiation, and h is

the angle of diffraction. For present case from different hvalues, calculated average size is about 8 nm. Inherent

stress inside a nanocrystal could contribute to broadening

of the XRD peaks. The XRD pattern indicates that the pure

copper selenide QDs were obtained under current synthetic

conditions.

Optical properties of the yielded QDs

To examine the optical properties of the yielded QDs, we

first perform room temperature UV–Visible absorp-

tion measurement. From the absorption spectra of the

as-synthesized QDs in Fig. 2, it is clear that the absorption

edge of the QDs shows a blue shift compared to that of its

bulk counterpart. The absorption spectra of copper selenide

QDs were studied without taking into account the reflection

and transmission losses. A broad absorption peak near

380 nm was observed that may originate due to excitonic

band edge and the confinement of electron and hole in very

small volume. From the absorption behavior of the QDs the

indirect and direct band gap energy (Eg) of QDs was found

to be 2.60 and 1.48 eV, respectively (Supporting infor-

mation available see Fig. S1), which showed large ‘‘blue

shift’’ from standard bulk band gap (Eg = 1.05 eV). The

blue shift might be caused by nanosize effect and structural

defects of QDs. Photoluminescence (PL) emission spectra

can be used to investigate the outcome of photo-generated

electron and holes in a semiconductor, since PL emis-

sion results from the recombination of free charge carriers.

A semiconductor is characterized by the electronic band

structure in which the highest occupied molecular orbital

called HOMO or valence band (VB), and the lowest

unoccupied molecular orbital called LUMO or conduction

band (CB). The energy difference between the HOMO and

LUMO level is regarded as band gap energy (Eg).

According to the attributes and formation mechanism of

PL, there are two types of PL phenomenon: the band–band

PL and the excitonic PL [45–49]. In general, the band–

band PL spectrum connects to the separation situation of

photo-generated charge carriers. But the excitonic PL

spectrum cannot directly reflect the separation situation of

photo-induced carriers. It can reveal some important

information about surface defects, vacancies, and surface

states, which strongly influence photocatalytic reactions.

We can utilize this information to find out the efficiency ofFig. 1 Powder XRD pattern of copper selenide QDs

Fig. 2 Absorbance spectra of copper selenide QDs at room temper-

ature. UV–Vis absorption spectra of copper selenide QDs at room

temperature

Struct Chem (2011) 22:103–110 105

123

charge carrier trapping, immigration, transfer, and to

understand the fate of electrons and holes in semiconduc-

tor. A broad emission PL spectrum, centered near 395 nm

of as-prepared sample at room temperature excited at

300 nm is shown in Fig. 3, which is attributed to the blue

emissions results. The colors of the nanoparticles result

because of the absorption in the visible region, and the

physical origin of this light absorption is the resonance of

the incident photon frequency with the collective oscilla-

tions of the conductive band gap electrons that is absent in

bulk material. Small peaks near 480 nm and at 500 nm are

due to defect state emission.

Morphological and topological studies

Figure 4a–d shows TEM micrographs of copper selenide

samples with different magnifications. From Fig. 4a–d of

copper selenide QDs it is raveled that the diameters of QDs

are in the range of 3–5 nm and they are well dispersed

without any aggregation. Figure 4d shows typical HRTEM

images of elongated and spherical copper selenide QDs at

the scale of 2 nm which revealed high crystallinity of

copper selenide QDs.

For topological and phase modulated studies, copper

selenide powder was deposited in the form of film on a

(1 9 1 inch and 1 mm thick) mica sheet by dispersing the

copper selenide powder in methanol. The topology of

as-deposited film surface is viewed two-dimensionally. In

phase imaging AFM, cantilever interaction with sample

surface is mapped by measuring change in phase angle

between approach and retraction. For stress free surfaces,Fig. 3 Photoluminescence spectra of the copper selenide sample at

room temperature

Fig. 4 TEM micrographs of

copper selenide QDs at different

magnifications

106 Struct Chem (2011) 22:103–110

123

phase contrast is due to change in chemical composition or

due to change in structure. Thin film samples were exposed

to white light using a 100 W quartz tungsten halogen

(QTH) lamp (with a spectral range of 450–850 nm), from a

distance of 10 cm for few hours at room temperature. In

order to obtain unbiased structural characterization data

measurements were done under ambient conditions (25 �C/

45% RH) using a micro-cantilever probe, tip radius and

probe spring constants in the range 5–10 nm and 4–5 N/m,

respectively. The samples were imaged using a 50 nm 9

50 nm and 30 nm 9 30 nm areas. Before each sample

observation, the condition of the cantilever was cross-

examined by taking images of the freshly cleaved mica

surface.

Figure 5a–d shows 2D topological and phase modulated

AFM images of as-deposited thin film. The AFM mor-

phology clearly revealed that the deposition of copper

selenide film took place via non-aggregations, excellent

dispersion of QDs, i.e., homogeneous precipitation of the

solution on mica sheet. QDs are well dispersed without

aggregations and are in very good agreement with the

values observed from TEM morphological studies. The

AFM image clearly shows the formation of copper selenide

QDs of nearly equal size.

Fig. 5 Topographical and phase modulated AFM images in (50 nm)2 and (30 nm)2 scan area of copper selenide QDs film prepared on mica

surface

Struct Chem (2011) 22:103–110 107

123

FTIR spectroscopy and proposed mechanism

Figure 6 shows the FTIR spectrum of as-prepared sample.

The sample was washed with absolute ethanol and hot

distilled water several times and then dried in vacuum

before used. We determined the IR absorption of the

obtained copper selenide powder with KBr. As can be seen

from Fig. 6, the broad peak at 3399 cm-1 and the peak at

1422 cm-1 are assigned to O–H characteristic vibrations

resulting from all small quantity of H2O on the sample. The

sharp peak at 2924 cm-1 corresponds to N–H stretching

vibration band and the shift toward lower frequency com-

pared with hydrazine may result from the interaction of

N2H4 with copper ion and regular periodic structure of

molecular precursor. Sharper and stronger peaks indicate

weaker interaction ordered arrangements of hydrazine

molecules existing in the precursors [50]. Though the

sample was washed with absolute ethanol and distilled

water several times, all IR characteristic indicates that the

N2H4 molecular has intercalated into the complex and

formed a molecular precursor directing the growth of

copper selenide QDs.

The exact mechanism for the formation of QDs is still

unclear, but it is reasonably concluded that the appropriate

ratio of solvents volume play the critical role for the for-

mation of QDs. Based on these observations, a reaction

mechanism of the copper selenide QDs is proposed. In this

study, Se source was derived from the reduction of Se by

N2H4. These highly reactive Se can be easily converted

into Se2-, which results in a high monomer concentration.

Fig. 6 FTIR analysis of

as-prepared sample milled in

KBr at room temperature

Fig. 7 Schematic representation of the formation of copper selenide

QDs

108 Struct Chem (2011) 22:103–110

123

In the initial step, hydrazine hydrate complexes with Cu2?

and forms the transparent soluble complexes solution,

which effectively decreases the concentration of Cu2? and

avoids the precipitation of CuSeO3, thus provides more

homogenous solution environment for the reaction. Se2- is

released slowly and interacts with surplus N2H4 to form the

molecular precursor immediately. The reaction could be

described as follows:

4Se2� þ 4N2H4 þ 4Cu2þ ! 4CuSe � N2H4

CuSe � N2H4 ! CuSeþ N2H4

So the decomposition of precursor can proceed thoroughly

under present condition. The application of N2H4 as the

coordination agent is determinal for the phase of the

products. So it can be drawn that the complexing ability of

groups containing atom N (such as NH2 or NH3) can

effectively determine the final phase of the products. All

the above results reveal that N2H4�H2O, as solvent, favors

the formation of copper selenide nanostructure. Based on

these observations, a growth scheme of copper selenide

QDs is proposed, which is shown in schematic represen-

tation in Fig. 7. Excess ethylene glycol and hydrazine

hydrate would be removed on heating the sample in vac-

uum followed by washing several times with ethanol and

hot distilled water.

Conclusions

In summary, we report on successful synthesis of lumi-

nescent and crystalline copper selenide QDs in aqueous

solution of hydrazine hydrate and ethylene glycol. Com-

pared with non-aqueous and substantiates used in prepa-

ration of other metal chalcogenides we present a simpler,

cheaper, and straightforward method which is free from

any capping agent, surfactant, or template. Strong ‘blue-

shifts’ were observed absorption and PL spectra which

could be attributed to the quantum confinement effect.

Hydrazine hydrate served not only as a complexing agent

and a molecular template but also as a reducing agent

which helps to dissolve Se in the mix solvent. TEM,

HRTEM, and AFM study confirm the formation of QDs.

The material is supposed to have very useful electrical,

optical, and thermal properties. We further expected that

the facile method of material synthesis described in this

article can be used in a broad range of applications to

fabricate innovative transition metal tellurides and sele-

nides (i.e., I–V and II–VI semiconductors) in nanocrystal-

line form having unique properties, avoiding use of high

temperature and costly methods. Further studies are needed

to investigate the effect and mechanism of broad absorp-

tion with blue shift.

Acknowledgments Pushpendra Kumar is grateful for support from

the University Grant Commission New Delhi for providing financial

assistance under Rajeev Gandhi National Fellowship Scheme as SRF

(RGNFS-SRF). We are also thankful to Prof. O.N. Srivastva,

Dr. Anchal Srivastva, Mr. Upendra Kumar Parashar (Dept. of Physics

BHU), Prof. Dhananjay Pandey (School of Materials Science,

IT-BHU), Dr. Avinash Chand Pandey, Mr. Vyom Parashar, and

Mr. Raghvendra S. Yadav (N.A.C. University of Allahabad) for

providing constant support and help in various ways.

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