Cds Nw Synthesis And Characterization.12

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

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 (20100nm). 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, 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 wide range 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= hν ) in falling from an excited state to

Figure 1. Generic energy band gap. 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

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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.

Figure 2. Direct band gap (GaAs) and indirect band gap (Si). 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 ( λ Excitation~517nm=5170Å). Alternatively, a current can be

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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 hν > 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 Planck’s constant (4.13566743 x 10-15 eV·s or 6.626 x 10-34 J·s) and ν is the frequency of the light (s-1). We also know c = λν , where c is the speed of light (299,792,458 m/s) and λ is the wavelength (m).

Name of Semiconductor Cadmium sulphide (CdS)

Band Gap (eV) at 300K 2.4

Wavelength Frequency (nm) (T Hz) 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.htm 13

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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 Å (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: http://www.thiel.edu/digitalelectronics/chapters/apph html/apph.htm 13

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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

Table 2. The colors of the visible light spectrum. Source: source: http://en.wikipedia.org/wiki/Color

1014 Hz

104 cm−1

kJ mol−1

Color

nm

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 Far ultraviolet

eV

300

10

3.33

4.15

400

<200

>15.0

>5.00

>6.20

>598

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

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 9

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.

Figure 6. Pseudo-binary Au-CdS phase diagram.

1Æ2

1Æ2

3

3

Figure 7. Nanowire growth. Figure 8. CdS vapor diffusion through Au catalyst for nanowire growth.

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

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pressure, the catalyst and its diameter, and the concentration of the CdS precursor (Cadmium Dimethylthiocarbonate). The

vapor-liquid-solid Manometers: Digital Gauge Analog Gauge

(VLS) method was used for the

Main Valve

fabrication of CdS nanowires.

Quartz Tube Ar Gas

This method has been reliably

Venting Valve

MFC

Tube Furnace

LN2 Trap

RP

used for over a decade for

Exhaust

Pressure displays and control

producing

one

dimensional

nanowires. VLS consists of two

MFC display and control

Figure 9. Horizontal LPCVD (low pressure chemical vapor deposition) schematic.

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

Tube Furnace Quartz Tube

CdS crystalline structure form nanowires.

to

one-dimensional By using colloidal

Au particles as the catalyst in this

Ar Flow

CdS Precursor: Cadmium Dimethylthiocarbonate Process Temp = 780°C

Si (100) Substrate Substrate Temp = 680°C

Figure 10. Loaded quartz tube schematic .

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

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microscopy (TEM), and high resolution transmission electron microscopy (HRTEM). For the compositional characterization of the nanowires, we used X-ray Energy dispersive spectroscopy (EDS).

MATERIALS AND METHODS Substrate Preparation •

Cut the Si <100> substrate into ~0.5 cm wide strip (Figure 11).

•

Rinse the Si substrate with acetone or ethyl alcohol (to remove organic materials).

•

Rinse with de-ionized water.

•

Blow dry with air.

•

Apply poly-L-lysine solution to clean substrate and leave 5-10 minutes. The poly-Llysine created a positive charge to aid in the adhesion of the nanoparticle gold (Au) catalyst.

•

Lightly blow dry with air.

•

Apply catalyst to substrate: colloidal gold nanoparticle solution to substrate (Au 2040 nm) with clean Pasteur pipette (Figure 12).

•

Lightly blow dry with air.

Figure 12. Application of Au catalyst.

Figure 11. Cutting Si <100> substrate. 12

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 14. Placing CdS precursor “boat”/ring into quartz tube. Figure 13. Placing CdS precursor into “boat”/ring.

Figure 15. Placing prepared quartz tube into furnace. 13

Glass wool

Precursor powder in “boat” Ar f

low

Ar f

Tube Furnace Substrate

low

Bolt to push precursor in slowly

Figure 16. Prepared loaded furnace.

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 750°C (typically ~800°C). The temperature at the edges of the furnace, input where the precursor “boat”/ring is and output where the substrate is placed is typically 70-100°C 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.

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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.

Pressure and vacuum displays and controls Valves

Argon Gas and Regulator

LN2 Trap

Tube Furnace (25-1100°C) Manometers: digital gauge (30-765 torr) and analog gauge (0-100 m torr)

Rotary (vacuum) Pump

MFC (100 SCCM)

Figure 17. LPCVD apparatus.

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RESULTS AND DISCUSSION Characterization – Structure - Imaging – Optical Microscope The nanostructures were first examined 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. Figure 18. Initial inspection of synthesized NWs with optical microscope.

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 Figure 19. Petri dish with multiple TEM grids.

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).

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Figure 20. TEM grid is mounted on tip of TEM assembly.

Figure 21. TEM assembly is inserted into TEM.

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 650°C. The CdS nanowires have diameters between 50-150nm and lengths up to 30µm as shown in the SEM images.

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

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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.

002

002

5 nm

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

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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 didn’t cause any alteration in the analysis of the graphs. S

CdS nanowire

?? Au Catalyst

20 nm

Molybdenum count spike due to Mo TEM grid

Figure 24. EDX/EDS images

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

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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 the CdS 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 Foundation’s (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 Lòpez (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.10:23012303.

[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. PolymerControlled 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," Nature 421, 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. 108:18826-18831. [15] L. Qu and X. Peng. 2002. Am Chem. Soc., 124: 2049.

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J. Phys. Chem B.