Synthesis and Characterization of Cadmium Sulfide (CdS) Nanowires (NWs) Edward Bujak and Dr. Ritesh Agarwal RET Program – University of Pennsylvania Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter University of Pennsylvania, PA, 19104-6272 July 27, 2006
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Synthesis and Characterization of
Cadmium Sulfide (CdS) Nanowires (NWs)
Edward Bujak and Dr. Ritesh Agarwal
RET Program – University of Pennsylvania
Department of Materials Science and Engineering
and
Laboratory for Research on the Structure of Matter
University of Pennsylvania, PA, 19104-6272
July 27, 2006
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ABSTRACT
Nanostructures have been investigated extensively using various compounds that
exhibit novel, peculiar, and fascinating properties in the nano scale not exhibited in the
bulk materials or superior to their bulk counterparts, such as: optical, electrical,
biological, mechanical, and chemical aspects, with various morphologies such as rods,
belts, ribbons, wire, helices, dots, and tubes. Dramatic progress has been made in the
investigation and application of these structures stimulating further research and
investment.
Semiconductor nanowires have been a focus of attention for nano-electronics and
nano-optics (or nano-optoelectronics). Specifically, cadmium sulfide (CdS) is a
semiconductor with a large and direct bandgap of Eg = 2.42 eV at room temperature
which, upon excitation, emits light of wavelength 517 nm (λ excitation~517nm). Due to
these unique properties, CdS is one of the most promising materials in optics devices.
This study’s main focus is on the synthesis and characterization of cadmium
sulfide nanowires (CdS NWs). Using conventional VLS growth, the NW synthesis was
performed with a custom made horizontal furnace chemical vapor deposition (CVD)
system. Colloidal Au nanoparticles were used as a catalyst with later studies using
sputtered Pt as a catalyst. The optimal condition for nanowire growth was established
varying process temperature, vacuum pressure, gas flow rate, and the diameter of the
catalyst. Characterization on morphology, crystal structure and chemical composition
were done using Optical microscopy, Scanning Electron microscopy (SEM),
Transmission Electron microscopy (TEM), High Resolution Transmission Electron
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microscopy (HRTEM), and X-ray Energy Dispersive Spectroscopy (EDS or EDX) in
STEM mode.
The morphology and the diameter of the nanowires were defined in controlled
fashion using different catalyst deposition methods and different sizes of catalyst (20-
100nm). We conclude that the dominant process parameter for optimal growth were the
temperature of the substrate and the concentration of the precursor. Further
characterization on optical properties is on the way.
INTRODUCTION
The field of electronics continues to grow and expand, but limits to progress are
falling to new and exciting possibilities. Microelectronics revitalized the fields of
telecommunication and technology through the bulk properties of materials in the
production of microchips and integrated circuits that contained millions of linked
semiconducting devices on the scale of µm (10-6 m). In the near future, nanoelectronic
devices may replace microelectronics in communication and computer industries with
nanostructures having one dimension between 1 and 100 nm.5 The emerging field of
nanoelectronics, electronics on the nanoscale, has the potential to take electronics, as well
as other fields, further than ever imagined. 1,11 This is possible because reducing the size
of a semiconductor to nanoscale proportions alters its bulk electronic, magnetic, and
optical properties.10 These enhanced properties enable multiple new applications
including the integration of nanomaterials into nanodevices such as biological imaging
and biolabeling14 , semiconducting nanowire high efficiency photovoltaic (PV) solar
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cells, waveguides, lasers, light emitting diodes (LED), optoelectronic devices, and a wide
array of photosensors, such as: photoresistors, photoconductive devices, photodetectors,
photodiodes, phototransistors, photodarlingtons, and slotted and reflective optical
switches.2 Various nanostructure morphologies have been synthesized such as: rods,
Figure 5. Normalized sensitivities of CdS, CdSe, and CdTe as a function of wavelength. Source: http://www.thiel.edu/digitalelectronics/chapters/apph html/apph.htm 13
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Table 3. Color, wavelength, frequency and energy of light source: http://en.wikipedia.org/wiki/Color
color wavelength interval frequency interval
red ~ 625–740 nm ~ 480–405 THz
orange ~ 590–625 nm ~ 510–480 THz
yellow ~ 565–590 nm ~ 530–510 THz
green ~ 500–565 nm ~ 600–530 THz
cyan ~ 485–500 nm ~ 620–600 THz
blue ~ 440–485 nm ~ 680–620 THz
violet ~ 380–440 nm ~ 790–680 THz
Color nm 1014 Hz 104 cm−1 eV kJ mol−1 Infrared >1000 <3.00 <1.00 <1.24 <120 Red 700 4.28 1.43 1.77 171 Orange 620 4.84 1.61 2 193 Yellow 580 5.17 1.72 2.14 206 Green 530 5.66 1.89 2.34 226 Blue 470 6.38 2.13 2.64 254 Violet 420 7.14 2.38 2.95 285 Near ultraviolet 300 10 3.33 4.15 400 Far ultraviolet <200 >15.0 >5.00 >6.20 >598
For the synthesis of CdS nanowires, it is necessary to understand the
thermodynamics of its formation. Figure 6 is the pseudo-binary phase diagram for gold
(Au) and cadmium sulfide (CdS) that illustrate the thermodynamics of vapor-liquid-solid
(VLS) growth. Note that this phase diagram shows that Au and CdS are partially soluble
in each other. This consists of the phases that pass the Au/CdS interface during the
temperature and time (concentrations) of the reaction. At a process temperature near
Table 2. The colors of the visible light spectrum. Source: source: http://en.wikipedia.org/wiki/Color
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800°C, the CdS in the vapor phase causes the solid nanoparticles (1) to form a liquid
alloy L (Au+CdS), and with an increasing concentration of CdS will cause a
supersaturation in the alloy (2), that will lead nucleation of the solid CdS growing the
nanowires. Figure 7 shows the diffusion process directly from the colloidal nanoparticles
of Au and the interaction with the CdS in the vapor phase.
In this study our major interest was the synthesis and characterization of
nanowires, especially cadmium sulfide. It was necessary to determine the optimal
parameters for the synthesis such as: process temperature, argon (Ar) flow rate, vacuum
1 2 3
Figure 7. Nanowire growth.
1 2 3
Figure 8. CdS vapor diffusion through Au catalyst for nanowire growth.
Figure 6. Pseudo-binary Au-CdS phase diagram.
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pressure, the catalyst and its diameter, and the concentration of the CdS precursor
(Cadmium Dimethylthiocarbonate).
The vapor-liquid-solid
(VLS) method was used for the
fabrication of CdS nanowires.
This method has been reliably
used for over a decade for
producing one dimensional
nanowires. VLS consists of two
main processes: evaporation and
condensation. Evaporation of the powder precursor is accomplished through high heat
(~800°C). Within a sealed quartz tube held at low pressure (~300 torr), the slowly
vaporizing precursor is carried through a by an inert Ar delivery gas to the Si <100>
substrate (~100 SCCM). The substrate is coated with a Au catalyst to stimulate the
nucleation and growth of the
CdS crystalline structure to
form one-dimensional
nanowires. By using colloidal
Au particles as the catalyst in this
technique, the morphology of the CdS nanowires growth is precisely controlled; the
synthesized nanowire diameters are the diameter of the colloidal Au particle. . The
process time was about 15 minutes. For the structural characterization of the nanowires,
we used optical microscopy, scanning electron microscopy (SEM), transmission electron