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Deposition method
January 2001
Materials Letters 47 Ž2001. 83–88 www.elsevier.comrlocatermatlet
Microstructure of nanocrystalline CdS powders and thin films by Electrostatic Assisted Aerosol Jet DecompositionrDeposition method B. Su ) , M. Wei, K.L. Choy Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK Received 26 November 1999; received in revised form 19 May 2000; accepted 14 June 2000
Abstract Nanocrystalline CdS powders and thin films have been prepared by an Electrostatic Assisted Aerosol Jet DecompositionrDeposition ŽEAAJD. method from an aqueous solution of cadmium chloride and thiourea. The microstructure of the powders and films was characterised by a combination of transmission electron microscopy ŽTEM., X-ray diffraction ŽXRD. and atomic force microscopy ŽAFM. techniques. The results showed that nanocrystalline powders and films were produced with a crystallite size less than 35 nm. A predominant hexagonal structure with a strong preferred orientation was formed in the films. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Nanocrystalline CdS powders and thin films; EAAJD
Nanocrystals of direct bandgap semiconductors such as CdS exhibit unusual optical properties which make them attractive as materials for nonlinear optical and photoluminescent devices w1x. This is because the nanocrystals or quantum dots lie in an interesting range of scale Že.g. typically 2–8 nm for II–VI semiconductors w2x.. Their electronic structure is between that of a molecule and a bulk material, which gives rise to profound modifications of the physical properties w3x. The size quantization effect, for example, allows the control of the colour of emitted light of the photoluminescent materials by adjusting the
) Corresponding author. IRC in Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
size of nanocrystals during synthesis. II–VI semiconducting quantum dots can either be added to a matrix to form composites or be used as matrix-free nanocrystalline semiconductor films. There are a number of ways to synthesis nanocrystalline powders and thin films. Among them, aerosol-based method is one of the most cost-effective and versatile methods w4x. We have developed a technique called Electrostatic Assisted Aerosol Jet Deposition ŽEAAJD. based on electrostatic charged aerosol w5,6x. In this method, aerosol was generated using an ultrasonic nebuliser and passed a corona unit to charge the aerosol particles. The electrostatic charged aerosol was then directed in an electric field towards the designed temperature gradient to enable the heterogeneous chemical reactions to occur in order to deposit uniform films or powders. We report here the
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 1 6 - 0
84
B. Su et al.r Materials Letters 47 (2001) 83–88
synthesis of CdS nanoparticles and nanocryslline thin films using the EAAJD technique. Their microstructural properties are characterised and discussed in terms of the electric field applied. Stoichiometric cadmium chloride ŽCdCl 2 . and thiourea wŽNH 2 . 2 CSx aqueous solution with molar concentrations of 0.005 to 0.01 M were used as the precursors for the EAAJD of CdS powders and thin films. The apparatus used in the EAAJD process has been described in Ref. w5x. Aerosols were generated using an ultrasonic aerosol generator at a frequency of 1.7 MHz with nitrogen as a carrier gas. The gas flow rate was 2 lrmin. An electrostatic nozzle with electric potential of 5 to 15 kV was used to discharge the aerosol. The charged aerosol was passed vertically through a heated zone during powder synthesis. Whereas, for the film deposition, the charged aerosol was directed towards the heated substrates under an electric field. The substrates used for film deposition were either uncoated or indium doped tin oxide ŽITO. coated optical glass slides. The deposition temperature was varied from 2008C to 4508C, and deposition time from 1 to 5 min. High resolution transmission electron microscopy ŽTEM. was employed for the charaterisation of nanocrystalline particles and films. The collected powders were suspended and de-agglomerated in an ultrasonic bath using acetone as the solvent for 5 to 10 min. The dispersed particles were then collected onto a 400-grid copper TEM grid coated with a thin amorphous carbon film. TEM specimens of CdS thin films were prepared using a standard thinning procedure, including mechanical thinning, dimpling Žby Gatan dimpler. and precisely ion polishing Žin PIPS, Gatan 691.. A JEOL 2000FX electron microscope Žoperation voltage 200 kV. was used for conventional TEM imaging and selected area electron diffraction ŽSAED.. The phase-contrast lattice images were recorded in a high-resolution transmission electron microscope ŽJEOL JEM2010.. A PHILIPS PW1710 X-ray diffraction spectrometer ŽCuK a radiation. was employed to study the phase and crystallinity of the powders and films. A QUESANT atomic force microscopy ŽAFM. was used to examine surface morphology and grain size of the films. Fig. 1 shows the X-ray diffraction ŽXRD. patterns of CdS films deposited at different temperatures. A strong peak at 2 u s 26.88 was observed for all the
Fig. 1. XRD patterns of CdS films deposited at different temperatures.
films deposited at different temperatures. This peak may be assigned to the Ž002. plane of hexagonal CdS or Ž111. plane of cubic CdS. However, from the absence of the characteristic Ž200. and Ž311. peaks of the cubic CdS, we may conclude that the EAAJD CdS films are predominately of hexagonal Žwurtzite. crystal structure and strong preferred orientation with c-axis normal to film surface. The crystal size calculated from Scherrer’s equation for CdS films deposited at different temperatures is shown in Fig. 2. The crystal size increases with deposition temperature. But the maximum crystal size is less than 35 nm at deposition temperatures below 4508C. Fig. 3 shows a plain view of TEM micrograph of a CdS thin film deposited at 3508C. The inset SAED pattern from the same area confirms the crystalline phase of the as-deposited CdS film is hexagonal wurzite structure. Fig. 4 gives a TEM image of the CdS particles deposited at 3508C with a HRTEM lattice image of a nanocrystalline CdS particle along Ž001. zone axis. The HRTEM lattice image shows clearly the hexagonal symmetry with a d-spacing of the Ž100. plane of 0.359 nm. The particle size distribution was measured from several TEM micrographs, as shown in Fig. 5. The average size of the CdS powders produced at 3508C was calculated to
B. Su et al.r Materials Letters 47 (2001) 83–88
Fig. 2. Crystal size calculated using Scherrer’s formula versus deposition temperature for CdS films.
be 9.9 nm with a standard deviation of 3.3 nm, which was close to the Scherrer estimation from the XRD results.
85
The surface morphology of the CdS films deposited at different temperatures is shown in Fig. 6. The grain size does not change significantly with the deposition temperature. It can be seen from Fig. 6 that the average grain size is ca. 80–200 nm, larger than the crystal size estimated from the Scherrer equation, which shows polycrystalline nature of the films. The grain size measured using the AFM technique is probably comprised of several crystallites and sometimes referred as the surface roughness. Bardet et al. w7x investigated the apparent discrepancy between microscopy and X-ray sizes in microcrystalline silicon. They concluded that the X-ray derived mean crystal size was the relevant size and corresponded to the smallest crystals observed in microscopy. Though the crystallite size was increased with the deposition temperature as shown in Fig. 2, the grain size is primarily determined by the aerosol droplet size and the concentration of the precursor solution. The grain growth at different film deposition temperatures is negligible probably because of the short deposition time used Ž- 5 min. in this work.
Fig. 3. A TEM image of the CdS thin films deposited at 4508C. Inset is the SAED pattern.
86
B. Su et al.r Materials Letters 47 (2001) 83–88
Fig. 4. A TEM image of the CdS particles deposited at 3508C. Inset is the HRTEM image of a hexagonal CdS nanoparticle.
In summary, nanocrystalline CdS powders and thin films have been prepared by a novel EAAJD method from aqueous solution of cadmium chloride
and thiourea. They are of predominantly hexagonal ŽWurtzite. structure. The CdS thin films demonstrated a strong preferred orientation with c-axis
Fig. 5. Size distribution of CdS nanoparticles produced at 3508C.
B. Su et al.r Materials Letters 47 (2001) 83–88
normal to the film surface, which may be attributed to the electric field applied during the film deposi-
87
tion. Tong et al. w8x reported that an electric field-induced orientation occurred during the early stages of
Fig. 6. AFM images of CdS films deposited at different temperatures.
88
B. Su et al.r Materials Letters 47 (2001) 83–88
Fig. 6 Ž continued ..
the structural transformations of gel-derived alumina precursors by the in-situ observation in the TEM. They suggested that dipolar units produced during decomposition and restructuring in the precursor were aligned by the electric field. The nanocrysatlline CdS thin films exhibited good optical and optic-electrical properties w9x, along with the processing advantages of cost-efficiency and high deposition rate, which makes them especially attractive for large scale applications such as window materials for solar cells.
Acknowledgements The authors wish to thank EPSRC-ROPA ŽGRr L73562. for financial support.
References w1x C.T. Tsai, D.S. Chu, G.L. Chen, S.L. Yang, J. Appl. Phys. 79 Ž1996. 9105. w2x M.L. Steigerwald, L.E. Brus, Ann. Rev. Mater. Sci. 19 Ž1989. 471. w3x D. Chakravorty, A.K. Giri, Chemistry of Advanced Materials, Oxford, Blackwell, 1993. w4x A. Gurav, T. Kodas, T. Pluym, Y. Xiong, Aerosol Sci. Technol. 19 Ž1993. 411. w5x B., Su, K.L. Choy, British Patent, Application No. 9900955.7 ŽJan. 1999.. w6x B. Su, K.L. Choy, Thin Solid Films 361 Ž2000. 102. w7x E. Bardet, J.E. Bouree, M. Cuniot, J. Dixmier, P. Elkaim, J. LeDuigou, A.R. Middya, J. Perrin, J. Non-Cryst. Solids 200 Ž1996. 867. w8x J. Tong, J.C. Whealey, L. Eyring, Ultramicroscopy 52 Ž1993. 388. w9x B. Su, K.L. Choy, Thin Solid Films 361 Ž2000. 102.
Materials Letters 47 Ž2001. 83–88 www.elsevier.comrlocatermatlet
Microstructure of nanocrystalline CdS powders and thin films by Electrostatic Assisted Aerosol Jet DecompositionrDeposition method B. Su ) , M. Wei, K.L. Choy Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK Received 26 November 1999; received in revised form 19 May 2000; accepted 14 June 2000
Abstract Nanocrystalline CdS powders and thin films have been prepared by an Electrostatic Assisted Aerosol Jet DecompositionrDeposition ŽEAAJD. method from an aqueous solution of cadmium chloride and thiourea. The microstructure of the powders and films was characterised by a combination of transmission electron microscopy ŽTEM., X-ray diffraction ŽXRD. and atomic force microscopy ŽAFM. techniques. The results showed that nanocrystalline powders and films were produced with a crystallite size less than 35 nm. A predominant hexagonal structure with a strong preferred orientation was formed in the films. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Nanocrystalline CdS powders and thin films; EAAJD
Nanocrystals of direct bandgap semiconductors such as CdS exhibit unusual optical properties which make them attractive as materials for nonlinear optical and photoluminescent devices w1x. This is because the nanocrystals or quantum dots lie in an interesting range of scale Že.g. typically 2–8 nm for II–VI semiconductors w2x.. Their electronic structure is between that of a molecule and a bulk material, which gives rise to profound modifications of the physical properties w3x. The size quantization effect, for example, allows the control of the colour of emitted light of the photoluminescent materials by adjusting the
) Corresponding author. IRC in Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
size of nanocrystals during synthesis. II–VI semiconducting quantum dots can either be added to a matrix to form composites or be used as matrix-free nanocrystalline semiconductor films. There are a number of ways to synthesis nanocrystalline powders and thin films. Among them, aerosol-based method is one of the most cost-effective and versatile methods w4x. We have developed a technique called Electrostatic Assisted Aerosol Jet Deposition ŽEAAJD. based on electrostatic charged aerosol w5,6x. In this method, aerosol was generated using an ultrasonic nebuliser and passed a corona unit to charge the aerosol particles. The electrostatic charged aerosol was then directed in an electric field towards the designed temperature gradient to enable the heterogeneous chemical reactions to occur in order to deposit uniform films or powders. We report here the
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 1 6 - 0
84
B. Su et al.r Materials Letters 47 (2001) 83–88
synthesis of CdS nanoparticles and nanocryslline thin films using the EAAJD technique. Their microstructural properties are characterised and discussed in terms of the electric field applied. Stoichiometric cadmium chloride ŽCdCl 2 . and thiourea wŽNH 2 . 2 CSx aqueous solution with molar concentrations of 0.005 to 0.01 M were used as the precursors for the EAAJD of CdS powders and thin films. The apparatus used in the EAAJD process has been described in Ref. w5x. Aerosols were generated using an ultrasonic aerosol generator at a frequency of 1.7 MHz with nitrogen as a carrier gas. The gas flow rate was 2 lrmin. An electrostatic nozzle with electric potential of 5 to 15 kV was used to discharge the aerosol. The charged aerosol was passed vertically through a heated zone during powder synthesis. Whereas, for the film deposition, the charged aerosol was directed towards the heated substrates under an electric field. The substrates used for film deposition were either uncoated or indium doped tin oxide ŽITO. coated optical glass slides. The deposition temperature was varied from 2008C to 4508C, and deposition time from 1 to 5 min. High resolution transmission electron microscopy ŽTEM. was employed for the charaterisation of nanocrystalline particles and films. The collected powders were suspended and de-agglomerated in an ultrasonic bath using acetone as the solvent for 5 to 10 min. The dispersed particles were then collected onto a 400-grid copper TEM grid coated with a thin amorphous carbon film. TEM specimens of CdS thin films were prepared using a standard thinning procedure, including mechanical thinning, dimpling Žby Gatan dimpler. and precisely ion polishing Žin PIPS, Gatan 691.. A JEOL 2000FX electron microscope Žoperation voltage 200 kV. was used for conventional TEM imaging and selected area electron diffraction ŽSAED.. The phase-contrast lattice images were recorded in a high-resolution transmission electron microscope ŽJEOL JEM2010.. A PHILIPS PW1710 X-ray diffraction spectrometer ŽCuK a radiation. was employed to study the phase and crystallinity of the powders and films. A QUESANT atomic force microscopy ŽAFM. was used to examine surface morphology and grain size of the films. Fig. 1 shows the X-ray diffraction ŽXRD. patterns of CdS films deposited at different temperatures. A strong peak at 2 u s 26.88 was observed for all the
Fig. 1. XRD patterns of CdS films deposited at different temperatures.
films deposited at different temperatures. This peak may be assigned to the Ž002. plane of hexagonal CdS or Ž111. plane of cubic CdS. However, from the absence of the characteristic Ž200. and Ž311. peaks of the cubic CdS, we may conclude that the EAAJD CdS films are predominately of hexagonal Žwurtzite. crystal structure and strong preferred orientation with c-axis normal to film surface. The crystal size calculated from Scherrer’s equation for CdS films deposited at different temperatures is shown in Fig. 2. The crystal size increases with deposition temperature. But the maximum crystal size is less than 35 nm at deposition temperatures below 4508C. Fig. 3 shows a plain view of TEM micrograph of a CdS thin film deposited at 3508C. The inset SAED pattern from the same area confirms the crystalline phase of the as-deposited CdS film is hexagonal wurzite structure. Fig. 4 gives a TEM image of the CdS particles deposited at 3508C with a HRTEM lattice image of a nanocrystalline CdS particle along Ž001. zone axis. The HRTEM lattice image shows clearly the hexagonal symmetry with a d-spacing of the Ž100. plane of 0.359 nm. The particle size distribution was measured from several TEM micrographs, as shown in Fig. 5. The average size of the CdS powders produced at 3508C was calculated to
B. Su et al.r Materials Letters 47 (2001) 83–88
Fig. 2. Crystal size calculated using Scherrer’s formula versus deposition temperature for CdS films.
be 9.9 nm with a standard deviation of 3.3 nm, which was close to the Scherrer estimation from the XRD results.
85
The surface morphology of the CdS films deposited at different temperatures is shown in Fig. 6. The grain size does not change significantly with the deposition temperature. It can be seen from Fig. 6 that the average grain size is ca. 80–200 nm, larger than the crystal size estimated from the Scherrer equation, which shows polycrystalline nature of the films. The grain size measured using the AFM technique is probably comprised of several crystallites and sometimes referred as the surface roughness. Bardet et al. w7x investigated the apparent discrepancy between microscopy and X-ray sizes in microcrystalline silicon. They concluded that the X-ray derived mean crystal size was the relevant size and corresponded to the smallest crystals observed in microscopy. Though the crystallite size was increased with the deposition temperature as shown in Fig. 2, the grain size is primarily determined by the aerosol droplet size and the concentration of the precursor solution. The grain growth at different film deposition temperatures is negligible probably because of the short deposition time used Ž- 5 min. in this work.
Fig. 3. A TEM image of the CdS thin films deposited at 4508C. Inset is the SAED pattern.
86
B. Su et al.r Materials Letters 47 (2001) 83–88
Fig. 4. A TEM image of the CdS particles deposited at 3508C. Inset is the HRTEM image of a hexagonal CdS nanoparticle.
In summary, nanocrystalline CdS powders and thin films have been prepared by a novel EAAJD method from aqueous solution of cadmium chloride
and thiourea. They are of predominantly hexagonal ŽWurtzite. structure. The CdS thin films demonstrated a strong preferred orientation with c-axis
Fig. 5. Size distribution of CdS nanoparticles produced at 3508C.
B. Su et al.r Materials Letters 47 (2001) 83–88
normal to the film surface, which may be attributed to the electric field applied during the film deposi-
87
tion. Tong et al. w8x reported that an electric field-induced orientation occurred during the early stages of
Fig. 6. AFM images of CdS films deposited at different temperatures.
88
B. Su et al.r Materials Letters 47 (2001) 83–88
Fig. 6 Ž continued ..
the structural transformations of gel-derived alumina precursors by the in-situ observation in the TEM. They suggested that dipolar units produced during decomposition and restructuring in the precursor were aligned by the electric field. The nanocrysatlline CdS thin films exhibited good optical and optic-electrical properties w9x, along with the processing advantages of cost-efficiency and high deposition rate, which makes them especially attractive for large scale applications such as window materials for solar cells.
Acknowledgements The authors wish to thank EPSRC-ROPA ŽGRr L73562. for financial support.
References w1x C.T. Tsai, D.S. Chu, G.L. Chen, S.L. Yang, J. Appl. Phys. 79 Ž1996. 9105. w2x M.L. Steigerwald, L.E. Brus, Ann. Rev. Mater. Sci. 19 Ž1989. 471. w3x D. Chakravorty, A.K. Giri, Chemistry of Advanced Materials, Oxford, Blackwell, 1993. w4x A. Gurav, T. Kodas, T. Pluym, Y. Xiong, Aerosol Sci. Technol. 19 Ž1993. 411. w5x B., Su, K.L. Choy, British Patent, Application No. 9900955.7 ŽJan. 1999.. w6x B. Su, K.L. Choy, Thin Solid Films 361 Ž2000. 102. w7x E. Bardet, J.E. Bouree, M. Cuniot, J. Dixmier, P. Elkaim, J. LeDuigou, A.R. Middya, J. Perrin, J. Non-Cryst. Solids 200 Ž1996. 867. w8x J. Tong, J.C. Whealey, L. Eyring, Ultramicroscopy 52 Ž1993. 388. w9x B. Su, K.L. Choy, Thin Solid Films 361 Ž2000. 102.