cds nw synthesis and characterization.12

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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 (excitation~517nm). Due to

these unique properties, CdS is one of the most promising materials in optics devices.

This studys 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 m (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,

belts, ribbons, spheres, helices, dots, tubes (single walled SWNT and double walled

(DWNT), branches (whiskers or dendritic), and core-shell (coaxial), to name a few, to

capitalize on their unique form and contour. In particular, semiconductor nanowires, in

which one dimension is approximately 100 times the other dimension, represent a broad

class of nanoscale building blocks that have been successfully used to assemble a widerange of electronic and photonic devices.

We study CdS because it has novel optical properties; namely its high

photoluminescence (PL) quantum efficiency.15 The energy band gap of CdS is direct

and large (wide). An electron will emit energy (E= h ) in falling from an excited state to

a ground state., but can fall directly or indirectly. With indirect band gap materials, the

electron in the conduction band moves to the point of energy minima at the expense of

Figure 1. Generic energy band gap.


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kinetic momentum. In indirect band gap materials, the electrons in the conductive band

need some source of momentum to reach the minimum and fall into the holes in the

valence band. With indirect energy band gap materials, the electron falls through one or

more intermediate energy bands so the emission of energy is gradual and an inefficient

source of light emission.

In a material with a direct energy band gap, such as CdS, the electron falls in one

step resulting in a faster, more concentrated emission of energy since the conductive band

is directly combined with the valence band, conserving kinetic energy. The energy that is

produced is emitted as a photon (light particle or quanta) and is therefore used in

applications such as solar cells and light-emitting diodes (LED). 1

The energy band gap of CdS is also large (wide) resulting in a relatively large

released energy than materials with a smaller energy band gap. The energy band gap for

CdS is 2.42eV (Eg = 2.42 eV ); corresponding to an excitation wavelength of

approximately 517 nm (Excitation~517nm=5170). Alternatively, a current can be

Figure 2. Direct band gap (GaAs) and indirect band gap (Si).


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measured when the CdS nanowires are exposed to light of wavelength smaller than 517

nm.

Technically the band gap is the energy difference between the valence band and

the conduction band or it is the energy required to break the chemical bonds thereby

producing free electrons and holes. From a practical point of view, the band gap energy

(Eg ) represents a lower limit on the photon energy necessary to cause a change in

resistance. Photons incident on these materials must have an energy h > Eg (or a lower

wavelength than its emitted wavelength) in order to cause a change in resistance. Eg is

the band gap in electron volts (eV), h is Plancks constant (4.13566743 x 10

-15

eVs or6.626 x 10-34 Js) and is the frequency of the light (s-1). We also know =c , where

c is the speed of light (299,792,458 m/s) and is the wavelength (m).

Name of Semiconductor

Band Gap

(eV) at 300K

Wavelength

(nm)

Frequency

(T Hz)

Cadmium sulphide (CdS) 2.4 517 580

Cadmium Phosphide (CdP) 2.2 564 532

Cadmium Selenide (CdSe) 1.7 729 411

Gallium Arsenide (GaAs) 1.4 886 338

Silicon (Si) 1.1 1127 266

Germanium (Ge) 0.7 1771 169

Indium Arsenide (InAs) 0.43 2883 104

Lead Sulphide (PbS) 0.37 3351 89

Lead Telluride (PbTe) 0.29 4275 70

Lead Selenide (PbSe) 0.26 4769 63

Indium Antimonide (InSb) 0.23 5390 56

Table 1. Photoresistive semiconductor materials.Derived from band gap data presented at

http://www.thiel.edu/digitalelectronics/chapters/apph_html/apph.htm13


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The peak sensitivity for photoresistors occurs at a frequency somewhat larger than

that determined by the band gap energy or equivalently at a wavelength somewhat

shorter than the wavelength determined by the band gap and falls off on either side. The

wavelength sensitivity for CdS, CdSe and CdTe normalized to a peak of 1 in each case is

shown in Figure 5. Note that the peak wavelength of CdS is at 5180 (518 nm) or a low

wavelength green

Figure 3. CdS Photoresistive detectors.


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Figure 4. Electromagnetic/Visible Spectrum.Source: http://en.wikipedia.org/wiki/Electromagnetic_spectrum

Figure 5. Normalized sensitivities of CdS, CdSe, and CdTe as a function of wavelength.Source: htt ://www.thiel.edu/di italelectronics/cha ters/a h html/a h.htm 13


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Table 3. Color, wavelength, frequency and energy of lightsource: http://en.wikipedia.org/wiki/Color

color wavelength interval frequency interval

red ~ 625740 nm ~ 480405 THz

orange ~ 590625 nm ~ 510480 THz

yellow ~ 565590 nm ~ 530510 THz

green ~ 500565 nm ~ 600530 THz

cyan ~ 485500 nm ~ 620600 THz

blue ~ 440485 nm ~ 680620 THz

violet ~ 380440 nm ~ 790680 THz

Color nm 1014

Hz 104

cm1

eV kJ mol1

Infrared >1000


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800C, 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

12 3

Figure 7. Nanowire growth.

12 3

Figure 8. CdS vapor diffusion through Aucatalyst for nanowire growth.

Figure 6. Pseudo-binaryAu-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 twomain processes: evaporation and

condensation. Evaporation of the powder precursor is accomplished through high heat

(~800C). 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

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

Quartz Tube

CdS Precursor:

Cadmium DimethylthiocarbonateProcess Temp = 780C

Si (100) SubstrateSubstrate Temp = 680C

Ar Flow

Tube Furnace

ArGas

RP

Tube Furnace

Quartz Tube

Manometers:

Analog Gauge

Main Valve

Venting Valve

Exhaust

MFCLN2 Trap

Digital Gauge

MFC display and control

Pressure displays and control

Figure 9. Horizontal LPCVD (low pressure chemicalva or de osition schematic.

Figure 10. Loaded quartz tube schematic .


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Quartz Tube Preparation

Under a chemical fume hood:

Place prepared substrate into end of quartz tube.

Load precursor into combustion boat/ring (Figure 13) and place into

opposite end of quartz tube with a steel bolt.

Place quartz tube into tube furnace, place glass wool at end of tube (Figures 15,16).

Figure 13. Placing CdSprecursor into boat/ring.

Figure 14. Placing CdS precursor

boat/ring into quartz tube.

Figure 15. Placing prepared quartz tube intofurnace.


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Fabrication/Synthesis of Nanowires

Install liquid nitrogen trap into system and fill with liquid nitrogen.

With vacuum pump:

Check vacuum of system (assure sustained 20 m torr vacuum test).

After integrity test (above), set operating low pressure vacuum (~300 torr).

Start Ar carrier gas flow (~100 SCCM).

Start tube furnace. The temperature of the process must be at least 750C (typically

~800C). The temperature at the edges of the furnace, input where the precursor

boat/ring is and output where the substrate is placed is typically 70-100C less. Once operating temperature is reached, slowly push the precursor boat/ring into the

furnace with a bolt moved by a magnet.

After a desired growth time (~15 minutes), stop tube furnace, let cool down.

Glass wool

Arflow

Arflow

Tube FurnaceBolt topushprecursorin slowly

Precursor

powder in

boat

Substrate

Figure 16. Prepared loaded furnace.


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After near room temperature, stop Ar flow, vent vacuum, and disassemble quartz tube

from furnace.

Under a chemical fume hood, remove substrate with grown nanowires and safely

dispose of all hazardous materials.

If this is last fabrication of the day, remove the liquid nitrogen trap and place in

chemical fume hood.

Manometers:

digital gauge (30-765torr) and analog

gauge (0-100 m torr)

Argon Gas and

Regulator

LN2 Trap

Rotary (vacuum)

Pump MFC

(100 SCCM)

Valves

Tube Furnace(25-1100C)

Pressure and vacuum

displays and controls

Figure 17. LPCVD apparatus.


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RESULTS AND DISCUSSION

Characterization Structure - Imaging Optical Microscope

The nanostructures were firstexamined directly on the Si substrate with

optical microscopes (Figure 18). If the

morphology and dimensions were

desirable, we then processed the nanowires

for electron microscopy.

Characterization Structure - Imaging - Electron Microscopy

(SEM/TEM/HRTEM)

We removed the good CdS

nanoparticles from the Si substrate by

scraping the particles off into a small vial,

mixed with acetone, and sonicated it to

disperse the particles uniformly in

suspension. With a Pasteur pipette we

placed drops of the processed nanoparticles

onto a TEM grid. We optionally make a few TEM grids and let air dry. We mounted the

TEM grid into the TEM scanning assembly (Figure 20) and placed it into the TEM

(Figure 21).

Figure 18. Initial inspection of synthesizedNWs with optical microscope.

Figure 19. Petri dish with multiple TEMgrids.


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The structures of the synthesized products were characterized using scanning

electron microscopy (SEM). Figure 22(a) and 22(b) shows the SEM images of the

nanowires grown on the Si at a temperature of 650C. The CdS nanowires have diameters

between 50-150nm and lengths up to 30m as shown in the SEM images.

Figure 20. TEM grid is mountedon tip of TEM assembly.

Figure 21. TEM assembly isinserted into TEM.

Figure 22. SEM images of CdS NWs grown in large scale.NW diameter: 50-150nm, length: up to 30 m.


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The morphology of CdS nanowires was observed in a transmission electron

microscope (TEM). Figure 23(a) is a typical TEM image, which demonstrates the general

view of the CdS nanowires. Figure 23(b) is a High-Resolution TEM (HRTEM) image

showing the uniformity of the grown nanowires. Figure 23(c) shows an equivalent image

of the CdS nanowires demonstrating the single crystalline nature.

Figure 23. (a) TEM image of CdS NWs (b) HRTEM image (c) Fourier Transform ofHRTEM image.

Characterization Composition Energy Dispersive X-Ray Spectroscopy

(EDX or EDS)

Energy Dispersive X-Ray Spectroscopy (EDX or EDS) analysis was utilized to

characterize the chemical composition of the nanowires. Figure 24 shows a diffraction

pattern of single-crystalline CdS nanowires. The graphs (counts on the y-axis for a certain

emitted Energy eV on the x-axis) demonstrate that the bodies of the nanowires arebasically composed of Cadmium with a peak between 0.0-5.0 eV and a peak of Sulfide in

the same range. It can be observed, in the second graph, that the tip of the nanowires is

5 nm

002002


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composed almost completely of gold (Au) with a peak between 0.0-5.0 eV and a wide

peak between 5.0-10.0 eV.

The high peak in the second graph is an artifact from the Molybdenum (Mo) TEM

grid. The peak, between 10.0-15.0 eV, is unknown but didnt cause any alteration in the

analysis of the graphs.

CONCLUSION

We successfully synthesized CdS nanowires under controlled conditions with an

established protocol utilizing a simple Vapor-Liquid-Solid mechanism at low pressure

(LPCVD) We observed different nanowire growth by varying process parameters such

as: temperature (700C-780C), time (5-10 minutes), vacuum pressure (~300 torr),

Molybdenum count spike due to Mo TEM grid

Figure 24. EDX/EDS images

S

??

Au Catalyst

CdS

nanowire

20 nm


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carrier-delivery flow (100-300 SCCM), and catalyst (Au ~20-40nm nanocolloidal

particles and sputtered Pt). We observed that the major factors affecting desirable

nanowire morphology and density were concentration of the vapor delivered to the

substrate/catalyst and the process temperature.

Structurally we imaged the fabricated CdS nanowires with scanning electron

microscope (SEM), transmission electron microscope (TEM, and high-resolution

transmission electron microscope (HRTEM) which occasionally showed good nanowire

morphologies of length to width: long and thin.

Compositionally we examined the purity, density, and chemical makeup of theCdS nanowire, utilizing Energy Dispersive X-Ray Spectroscopy (EDS or EDX) which

showed a uniform and high concentration of Cd and S across the nanowires with little or

no Au or Pt catalyst. Similarly the catalyst at the tip of the nanowires was effectively pure

Au or Pt and do not show any Cd or S peaks.

Optically, CdS is a very interesting photoluminescence (PL) material.

Unfortunately in this time frame we did not have time to investigate these. I understand

what is needed to realize this, but it was a limitation of available equipment.

ACKNOWLEDGEMENTS

I would like to thank the National Science Foundations (NSF) funding of the

Research Experience for Teachers (RET) and Dr. Andrew McGhie for this wonderful

opportunity to provide me and other teachers an experience in advanced exploratory

scientific discovery and research. Specifically I would like to thank my advisor Dr.

Ritesh Agarwal and his group of graduate and post doctoral students: Dr. Se-Ho Lee


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(Lee), Yeonwoong Jung (Eric), Dong-Kyun Ko (Ko), Yu-Han Cheng (Valorie), Xuelian

Zhu (Julian), and undergraduate student Andrew Jennings. Furthermore I would like to

thank fellow visiting individuals in this group: Maria Lpez (REU-Research Experience

for Undergraduates, University of Puerto Rico), Dr. Spirit Tlali (Collaborative with

Southern Africa Lesotho), and Dr. Murrell Dobbins (RET-Nanotechnology-Drexel

University) for their assistance and support.

REFERENCES

[1] J.H. Zhan, X.G. Yang, S.D. Li, D.W. Wang, Y. Xie and Y.T. Qian. 2000. A

chemical solution transport mechanism for one-dimensional growth of CdS

nanowires.Journal of Crystal Growth.220:231-234.

[2] C. Barrelet, Y. Wu, D. Bell and C. Lieber. 2003. Synthesis of CdS and ZnS

Nanowires Using Single-Source Molecular Precursor.

J.Am.Chem.Soc.125:11498-11499.

[3] Y. Li, H. Liao, Y. Ding, Y. Qian, L Yang and G. Zhou. 1998. Nonaqueous

Synthesis of CdS Nanorod Semiconductor. Chemistry of Materials.10:2301-

2303.

[4] R.S. Wagner and W.C. Ellis. 1964. Vapor-Liquid-Solid Mechanism of Single

Crystal Growth.Appl. Physics Letters.4: 89-90.

[5] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim and H. Yan.

2003. One-Dimensional Nanostructures: Synthesis, Characterization and

Applications.Adv.Mater.15:353-389.


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[6] L.J. Lauhon, M. Gudiksen and C. Lieber. 2004. Semiconductor Nanowire

heterostructure. Phil. Trans. R. Soc. Lond. A. 362:1247-1260.

[7] C. M. Lieber / Phil. Trans. R. Soc. Lond. A (2004)

[8] J. Zhan, X. Yang, D. Wang, S. Li, Y. Xie, Y. Xia and Y. Qian. 2000. Polymer-

Controlled Growth of CdS Nanowires.Adv. Mater.12:1348-1351.

[9] J. Joswig, G. Seifeit, T. Niehaus and M. Springbord. 2003. Optical properties of

Cadmium Sulfide Cluster.J. Phys.Chem.107:2897-2902.

[10] J. Milam, L. Lauhon and J. Allen. 2005.Photoconductivy of Semiconducting CdS

Nanowires.Nanoscape. 2:43-47.[11] C. M. Lieber / Sci. Am. 285 (2001).

[12] R. Agarwal, C. J. Barrelet and C. M. Lieber, "Lasing Mechanism in Single

Cadmium Sulfide Nanowire Optical Cavities,"Nano Lett. 5, 917-920 (2005).

[13] X. Duan, Y. Huang, R. Agarwal, and C.M. Lieber, "Single-Nanowire Electrically

Driven Lasers,"Nature421, 241 (2003).

[13] Interactive Digital Electronics (on line), Appendix H Sensors/Transducers,

http://www.thiel.edu/digitalelectronics/chapters/apph_html/apph.htm, part of

http://www.thiel.edu/digitalelectronics/, July 25, 2006

[14] D. V. Talapin, I. Mekis, St. Gotzinger, and A. Kornowski. 2004. CdSe/CdS/ZnS

and CdSe/ZnSe/ZnS Core-Shell-Shell Nanocrystals. J. Phys. Chem B.

108:18826-18831.

[15] L. Qu and X. Peng. 2002. Am Chem. Soc., 124: 2049.

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