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Chemical passivation of sputter-deposited nanocrystalline CdS thin films
June 2002
Materials Letters 54 (2002) 343 – 347 www.elsevier.com/locate/matlet
Chemical passivation of sputter-deposited nanocrystalline CdS thin films Praveen Taneja, Parinda Vasa, Pushan Ayyub* Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India Received 4 February 2001; accepted 10 July 2001
Abstract Due to their extremely high surface and interface area, nanocrystalline materials are extraordinarily susceptible to environmental degradation, and often need to be effectively isolated from the ambient before they can be put to applications. In addition, semiconductor nanoparticles usually require to be surface-passivated in order to enhance radiative decay cross sections. We describe an rf-sputtering technique for the synthesis of nanocrystalline CdS thin films and propose a simple and effective method for the in-situ surface passivation of such films by terminating the surface dangling bonds of CdS with hydrogen. D 2002 Elsevier Science B.V. All rights reserved. PACS: 81.07.-b; 81.40.Tv; 81.65.Rv; 81.15.Cd Keywords: Nanocrystals; Thin films; Semiconductors; Cadmium sulfide; Chemical passivation; Sputtering
1. Introduction During the past decade, there has been a surge in activity in the field of nanocrystalline materials of various types, but particularly in semiconductor nanoparticles. This is mainly because particle size reduction in semiconductors leads to a very significant blue shift in the band gap [1] as well as an enhancement in oscillator strengths [2] of various optical transitions. This has found important applications in optoelectronic devices [3,4], lasers [5], fluorescent labeling of cell organelles [6], etc.
*
Corresponding author. Tel.: +91-22-215-2971x2295; fax: +91-22-215-2110. E-mail address: [email protected] (P. Ayyub).
A native semiconductor nanoparticle can have many surface electronic states in the band gap region, which act as traps for electrons and holes and lead to a severe degradation in the optical and electrical properties. To overcome this problem in the case of particles synthesized in liquid media, capping agents are added either during or after nanoparticle formation. Different types of wet chemical synthesis techniques based on precipitation [7,8], sol – gel [9], colloidal systems [10,11], etc. have been developed for this purpose. Surface passivation, using organic [12,13] as well as inorganic [14,15] capping agents, has been successfully accomplished. Nonradiative recombination from surface states in CdS, for example, can be reduced by coating with TOP/TOPO [12] or Cd(OH)2 [14]. However, for most device applications, it is required to first coat these suspended semiconductor particles in a
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 5 9 0 - 0
344
P. Taneja et al. / Materials Letters 54 (2002) 343–347
liquid medium, and then form a thin film on a desired substrate. Methods such as dip coating and spin coating are often used for this purpose. For many types of applications, the advantages of using physical vapour deposition (PVD) techniques are becoming increasingly clear. Using, for example, dc or rf sputtering at relatively low temperatures and high inert gas pressures, one can produce nanocrystalline thin films of different types of materials with a reasonably well-controlled particle size and stoichiometry [16,17]. The need for a post-annealing or calcination step—commonly required in most wet chemical techniques—is obviated in this case. This is very useful, since such thermal treatment usually leads to a coarsening in the particle size, and a broadening of the size distribution, in addition to undesirable chemical reactions. It would be extremely advantageous if one could modify such PVD techniques to directly obtain a surface-passivated nanocrystalline thin film on a substrate, at or below room temperature. One option that has been utilized with some success is to co-sputter the material of interest and the capping material, in appropriate proportions. We report here the synthesis as well as in-situ chemical passivation of sputter-deposited nanocrystalline CdS thin films. The passivation is achieved by an exposure to hydrogen immediately after sputtering. In the absence of this step, the nano-CdS was found to undergo atmospheric corrosion almost immediately on removing from the sputtering chamber. However, if the hydrogen passivation is done, the nano-CdS films remain chemically stable and retain their photoluminescence and other optical properties.
2. Experimental details High-pressure rf magnetron sputtering was used to deposit nanocrystalline thin films of CdS on quartz plates or silicon wafers. Sputtering was done in an ambient of flowing Ar, whose pressure was maintained at f0.1 mbar. At the relatively high ambient gas pressure, the sputtered species are expected to undergo multiple collisions and arrive at the substrate with energies much lower than in conventional sputtering. Since the substrate itself is maintained at a relatively low temperature (100 – 300 K), sufficient
Table 1 Processing parameters for the nanocrystalline CdS samples Sample name
Argon rf power Gas Sample dXRD gas pressure (W) treatment degradation (nm) (mbar)
Sample 1 5.010 Sample 2 1.010 Sample 3 1.010
3 1 1
90 90 90
none absent none present H2 (20 h) absent
17 – 2.7
energy is not available for the deposited atoms to grow into large crystallites. The sputtering target consisted of a 50-mm diameter disc of compressed CdS powder (99.99% pure). A Siemens D-500 powder X-ray diffractometer (XRD) was used for crystallographic phase identification as well as particle size analysis. The surface features of the chemically degraded and the passivated films were studied by a metallographic microscope (Neophot 21, Carl Zeiss). Photoluminescence spectra (at room temperature) were obtained using a 0.22-m spectrometer (Spex 1681). The crystallographic domain size (dXRD) of the nanocrystalline CdS thin films was determined from a measurement of the integral line width in the X-ray diffraction spectra using the Scherrer technique (after subtracting the Ka2 contribution and correcting for instrumental broadening). The particle size of the deposited nanocrystalline CdS was controlled by changing the pressure of the inert gas (Ar) forming the plasma. Highly oriented thin films, with preferred growth along the (002) direction, were formed at low Ar pressures. At higher pressures, the films showed only two broad humps in the XRD spectra, indicating a very small particle size of CdS (dXRDc2 nm). Table 1 summarizes the synthesis parameters for three representative samples.
3. Results and discussion One motivation for this study was our observation that sputter-deposited nanocrystalline thin films of CdS develop a pitted texture and a greenish appearance very soon after being removed from the sputtering chamber and exposed to the atmosphere. X-ray diffraction analysis of these films shows the presence
P. Taneja et al. / Materials Letters 54 (2002) 343–347
of a large number of very sharp lines arising from impurity phases. When observed under high magnification using an optical microscope, these films show the presence of brightly colored grains with sharp crystalline facets. Clearly, the nanocrystalline CdS thin films are highly susceptible to atmospheric degradation. This type of surface chemical reaction occurred more prominently when the films were deposited at a very high Ar pressure, and consequently had a smaller particle size. Samples 1, 2 and 3 are three representative types of nano-CdS thin films that were deposited by rf-sputtering (Table 1). Sample 1 was deposited at a very low Ar pressure (5.010 3 mbar), while Samples 2 and 3 were deposited at much higher pressure (1.010 1 mbar). However, while Sample 3 was held in an atmosphere of hydrogen gas (at 0.5 bar) for 20 h after deposition, the other samples were removed from the vacuum chamber soon after the deposition was over. Surface passivation was found to be equally effective in the presence of a gas mixture containing 10% H2+90% N2. Fig. 1(a) shows the XRD spectrum of Sample 1. The intense line that appears at 2h c26j is the (002) reflection from hexagonal CdS. Since the next strongest reflection is from (004), we conclude that the sample has a preferred growth along the c-direction. There is no evidence for any atmospheric chemical degradation of this sample. The value of dXRD calculated from the (002) line is 17 nm. In contrast to Sample 1, the XRD spectrum of Sample 2 shows a number of lines that do not match with the XRD pattern of either hexagonal or cubic CdS (Fig. 1(b)). Further, the XRD lines from this sample are very sharp and narrow, indicating the presence of large crystalline particles in the film. All the extra lines cannot be attributed to a single, crystalline impurity phase, but probably arise from hydrated cadmium sulfate and similar compounds. The XRD spectrum from Sample 3 (Fig. 1(c)) shows a broad hump centered at 2hc27.3j, which could arise from an overlap of the first three reflections from hexagonal CdS, broadened due to small particle size. However, considering the symmetric nature of both the broad peaks, it appears more probable that they originate from the cubic (zinc blende) phase of CdS. Assuming the observed feature to originate from the cubic (111) line, we obtain dXRD=2.7 nm. The ‘‘normal’’
345
Fig. 1. Powder X-ray diffraction spectra for sputter-deposited CdS thin films on Si. (a) Sample 1: CdS particles with a comparatively large particles size (dXRD=17 nm), which were not surfacepassivated but did not undergo atmospheric degradation. The broad ‘‘amorphous’’ hump near 20j comes from the glass substrate. (b) Sample 2: Unpassivated nanocrystalline CdS, showing sharp lines from crystalline impurity phases. (c) Sample 3: Hydrogenpassivated nanocrystalline CdS (dXRD=2.7 nm), showing no atmospheric degradation.
wurtzite phase of CdS is in fact known [18] to undergo a size-induced structural phase transition to the zinc blende phase when the particle size is c2 – 3 nm. The XRD studies therefore indicate that only Sample 2 undergoes a drastic and rapid chemical degradation, while the other two samples do not appear to be affected by atmospheric exposure. We now examine this conclusion in the light of the optical micrographic studies of the surfaces of the films. Fig. 2(a) shows the surface features of the degraded sample (Sample 2). At lower magnification one clearly observes strongly faceted, bright green and
346
P. Taneja et al. / Materials Letters 54 (2002) 343–347
Fig. 2. Optical micrographs (true color) of the sputter-deposited nanocrystalline CdS thin films: (a) Sample 2, and (b) Sample 3.
light yellow crystalline grains, while it is clearer at higher magnification (not shown) that these crystallites show a stratified growth pattern. In sharp contrast, the surface of Sample 3 shows no visible degradation at any magnification (Fig. 2(b)). The scratch on the surface of this sample was made deliberately, to ensure sharp focusing of the surface of the film. This film is essentially smooth and featureless at optical magnifications. The micro-
graphic image of Sample 1 is very similar to that of Sample 3 and is not shown. A comparison of the X-ray and optical studies of Samples 1 and 3 clearly indicates that the chemical reactivity with ambient gases increases with a reduction in the average particle size of the nanocrystalline CdS thin films. This is only to be expected because of the larger specific surface area and the consequent presence of a relatively larger number of chemically
P. Taneja et al. / Materials Letters 54 (2002) 343–347
347
4. Conclusion We describe an rf-sputtering technique for producing nanocrystalline CdS thin films. When the particle size is very small (c2 nm), the nano-CdS undergoes chemical reaction with atmospheric components and gets rapidly degraded. Here we have reported a simple and viable technique for the long-term chemical passivation of such CdS films. This was accomplished by exposing the sputter-deposited nanocrystalline samples in-situ to pure hydrogen gas (or a hydrogen/nitrogen gas mixture) for about 20 h. No chemical degradation was detected on examining these films optically and by X-ray diffraction, even 6 months after the synthesis and passivation. The passivated nanocrystalline CdS films show strong, blue-shifted luminescence excitation and emission peaks at room temperature. Fig. 3. Photoluminescence spectra recorded at room temperature from Sample 3. The excitation spectrum (emission=437 nm) appears on the low-wavelength end while the emission spectrum (excitation=367 nm) appears at higher wavelengths. The emission from the bare Si substrate is shown by the dotted line at the bottom, on the same scale.
active surface sites in the smaller particles. A comparison of the data from Samples 2 and 3 shows that such surface chemical sites can be passivated by bonding with hydrogen gas, thereby reducing the reactivity of nanocrystalline CdS. Fig. 3 shows the photoluminescence spectrum from the hydrogen-passivated nanocrystalline CdS thin film (Sample 3) recorded at room temperature. The source and monochromator slits were both kept at 1 mm. The figure shows the excitation spectrum recorded at an emission wavelength of 437 nm, and the emission spectrum recorded with the excitation wavelength kept fixed at 367 nm (which corresponds to the peak in the excitation spectrum). The emission scan from the bare Si substrate is also shown. The peak excitation and the peak emission wavelengths are both appreciably blue-shifted with respect to the wavelength corresponding to the band gap energy in the bulk state (2.42 eV or 515 nm). The blue shift in the band-gap energy is related to quantum confinement effects in semiconductor nanoparticles.
References [1] L.E. Brus, Appl. Phys. A 53 (1991) 465. [2] A.P. Alivisatos, Science 271 (1996) 933. [3] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [4] B.O. Dabbousi, M.G. Bawendi, O. Onitsuka, App. Phys. Lett. 66 (1995) 1316. [5] L. Pavesi, L.D. Negro, C. Mazzoleni, G. Franzo, F. Priolo, Nature 408 (2000) 440. [6] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [7] Y. Wang, N. Herron, J. Phys. Chem. 91 (1987) 257. [8] B.G. Potter, J.H. Simmons, Phys. Rev. B 37 (1988) 10838. [9] H. Mathieu, T. Richard, J. Allegre, P. Lefebvre, G. Arnaud, J. Appl. Phys. 77 (1995) 287. [10] N. Chestnoy, T.D. Harris, R. Hull, L.E. Brus, J. Phys Chem. 90 (1986) 3393. [11] R. Rossetti, R. Hull, J.M. Gibson, L.E. Brus, J. Chem. Phys. 82 (1985) 552. [12] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [13] D.J. Norris, A. Sacra, C.B. Murray, M.G. Bawendi, Phys. Rev. Lett. 72 (1994) 2612. [14] L. Spanhel, M. Haase, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 5649. [15] A.R. Kortan, R. Hull, R.L. Opila, M.G. Bawendi, M.L. Steigerwald, P.J. Carroll, L.E. Brus, J. Am. Chem. Soc. 112 (1990) 1327. [16] R. Chandra, P. Taneja, P. Ayyub, G.K. Dey, S.K. Kulshreshtha, Nanostruct. Mater. 11 (1999) 1171. [17] P. Ayyub, R. Chandra, P. Taneja, A.K. Sharma, R. Pinto, Appl. Phys. A 73 (2001) 67. [18] R. Banerjee, R. Jayakrishnan, P. Ayyub, J. Phys.: Condens. Matter 12 (2000) 10647.
Materials Letters 54 (2002) 343 – 347 www.elsevier.com/locate/matlet
Chemical passivation of sputter-deposited nanocrystalline CdS thin films Praveen Taneja, Parinda Vasa, Pushan Ayyub* Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India Received 4 February 2001; accepted 10 July 2001
Abstract Due to their extremely high surface and interface area, nanocrystalline materials are extraordinarily susceptible to environmental degradation, and often need to be effectively isolated from the ambient before they can be put to applications. In addition, semiconductor nanoparticles usually require to be surface-passivated in order to enhance radiative decay cross sections. We describe an rf-sputtering technique for the synthesis of nanocrystalline CdS thin films and propose a simple and effective method for the in-situ surface passivation of such films by terminating the surface dangling bonds of CdS with hydrogen. D 2002 Elsevier Science B.V. All rights reserved. PACS: 81.07.-b; 81.40.Tv; 81.65.Rv; 81.15.Cd Keywords: Nanocrystals; Thin films; Semiconductors; Cadmium sulfide; Chemical passivation; Sputtering
1. Introduction During the past decade, there has been a surge in activity in the field of nanocrystalline materials of various types, but particularly in semiconductor nanoparticles. This is mainly because particle size reduction in semiconductors leads to a very significant blue shift in the band gap [1] as well as an enhancement in oscillator strengths [2] of various optical transitions. This has found important applications in optoelectronic devices [3,4], lasers [5], fluorescent labeling of cell organelles [6], etc.
*
Corresponding author. Tel.: +91-22-215-2971x2295; fax: +91-22-215-2110. E-mail address: [email protected] (P. Ayyub).
A native semiconductor nanoparticle can have many surface electronic states in the band gap region, which act as traps for electrons and holes and lead to a severe degradation in the optical and electrical properties. To overcome this problem in the case of particles synthesized in liquid media, capping agents are added either during or after nanoparticle formation. Different types of wet chemical synthesis techniques based on precipitation [7,8], sol – gel [9], colloidal systems [10,11], etc. have been developed for this purpose. Surface passivation, using organic [12,13] as well as inorganic [14,15] capping agents, has been successfully accomplished. Nonradiative recombination from surface states in CdS, for example, can be reduced by coating with TOP/TOPO [12] or Cd(OH)2 [14]. However, for most device applications, it is required to first coat these suspended semiconductor particles in a
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 5 9 0 - 0
344
P. Taneja et al. / Materials Letters 54 (2002) 343–347
liquid medium, and then form a thin film on a desired substrate. Methods such as dip coating and spin coating are often used for this purpose. For many types of applications, the advantages of using physical vapour deposition (PVD) techniques are becoming increasingly clear. Using, for example, dc or rf sputtering at relatively low temperatures and high inert gas pressures, one can produce nanocrystalline thin films of different types of materials with a reasonably well-controlled particle size and stoichiometry [16,17]. The need for a post-annealing or calcination step—commonly required in most wet chemical techniques—is obviated in this case. This is very useful, since such thermal treatment usually leads to a coarsening in the particle size, and a broadening of the size distribution, in addition to undesirable chemical reactions. It would be extremely advantageous if one could modify such PVD techniques to directly obtain a surface-passivated nanocrystalline thin film on a substrate, at or below room temperature. One option that has been utilized with some success is to co-sputter the material of interest and the capping material, in appropriate proportions. We report here the synthesis as well as in-situ chemical passivation of sputter-deposited nanocrystalline CdS thin films. The passivation is achieved by an exposure to hydrogen immediately after sputtering. In the absence of this step, the nano-CdS was found to undergo atmospheric corrosion almost immediately on removing from the sputtering chamber. However, if the hydrogen passivation is done, the nano-CdS films remain chemically stable and retain their photoluminescence and other optical properties.
2. Experimental details High-pressure rf magnetron sputtering was used to deposit nanocrystalline thin films of CdS on quartz plates or silicon wafers. Sputtering was done in an ambient of flowing Ar, whose pressure was maintained at f0.1 mbar. At the relatively high ambient gas pressure, the sputtered species are expected to undergo multiple collisions and arrive at the substrate with energies much lower than in conventional sputtering. Since the substrate itself is maintained at a relatively low temperature (100 – 300 K), sufficient
Table 1 Processing parameters for the nanocrystalline CdS samples Sample name
Argon rf power Gas Sample dXRD gas pressure (W) treatment degradation (nm) (mbar)
Sample 1 5.010 Sample 2 1.010 Sample 3 1.010
3 1 1
90 90 90
none absent none present H2 (20 h) absent
17 – 2.7
energy is not available for the deposited atoms to grow into large crystallites. The sputtering target consisted of a 50-mm diameter disc of compressed CdS powder (99.99% pure). A Siemens D-500 powder X-ray diffractometer (XRD) was used for crystallographic phase identification as well as particle size analysis. The surface features of the chemically degraded and the passivated films were studied by a metallographic microscope (Neophot 21, Carl Zeiss). Photoluminescence spectra (at room temperature) were obtained using a 0.22-m spectrometer (Spex 1681). The crystallographic domain size (dXRD) of the nanocrystalline CdS thin films was determined from a measurement of the integral line width in the X-ray diffraction spectra using the Scherrer technique (after subtracting the Ka2 contribution and correcting for instrumental broadening). The particle size of the deposited nanocrystalline CdS was controlled by changing the pressure of the inert gas (Ar) forming the plasma. Highly oriented thin films, with preferred growth along the (002) direction, were formed at low Ar pressures. At higher pressures, the films showed only two broad humps in the XRD spectra, indicating a very small particle size of CdS (dXRDc2 nm). Table 1 summarizes the synthesis parameters for three representative samples.
3. Results and discussion One motivation for this study was our observation that sputter-deposited nanocrystalline thin films of CdS develop a pitted texture and a greenish appearance very soon after being removed from the sputtering chamber and exposed to the atmosphere. X-ray diffraction analysis of these films shows the presence
P. Taneja et al. / Materials Letters 54 (2002) 343–347
of a large number of very sharp lines arising from impurity phases. When observed under high magnification using an optical microscope, these films show the presence of brightly colored grains with sharp crystalline facets. Clearly, the nanocrystalline CdS thin films are highly susceptible to atmospheric degradation. This type of surface chemical reaction occurred more prominently when the films were deposited at a very high Ar pressure, and consequently had a smaller particle size. Samples 1, 2 and 3 are three representative types of nano-CdS thin films that were deposited by rf-sputtering (Table 1). Sample 1 was deposited at a very low Ar pressure (5.010 3 mbar), while Samples 2 and 3 were deposited at much higher pressure (1.010 1 mbar). However, while Sample 3 was held in an atmosphere of hydrogen gas (at 0.5 bar) for 20 h after deposition, the other samples were removed from the vacuum chamber soon after the deposition was over. Surface passivation was found to be equally effective in the presence of a gas mixture containing 10% H2+90% N2. Fig. 1(a) shows the XRD spectrum of Sample 1. The intense line that appears at 2h c26j is the (002) reflection from hexagonal CdS. Since the next strongest reflection is from (004), we conclude that the sample has a preferred growth along the c-direction. There is no evidence for any atmospheric chemical degradation of this sample. The value of dXRD calculated from the (002) line is 17 nm. In contrast to Sample 1, the XRD spectrum of Sample 2 shows a number of lines that do not match with the XRD pattern of either hexagonal or cubic CdS (Fig. 1(b)). Further, the XRD lines from this sample are very sharp and narrow, indicating the presence of large crystalline particles in the film. All the extra lines cannot be attributed to a single, crystalline impurity phase, but probably arise from hydrated cadmium sulfate and similar compounds. The XRD spectrum from Sample 3 (Fig. 1(c)) shows a broad hump centered at 2hc27.3j, which could arise from an overlap of the first three reflections from hexagonal CdS, broadened due to small particle size. However, considering the symmetric nature of both the broad peaks, it appears more probable that they originate from the cubic (zinc blende) phase of CdS. Assuming the observed feature to originate from the cubic (111) line, we obtain dXRD=2.7 nm. The ‘‘normal’’
345
Fig. 1. Powder X-ray diffraction spectra for sputter-deposited CdS thin films on Si. (a) Sample 1: CdS particles with a comparatively large particles size (dXRD=17 nm), which were not surfacepassivated but did not undergo atmospheric degradation. The broad ‘‘amorphous’’ hump near 20j comes from the glass substrate. (b) Sample 2: Unpassivated nanocrystalline CdS, showing sharp lines from crystalline impurity phases. (c) Sample 3: Hydrogenpassivated nanocrystalline CdS (dXRD=2.7 nm), showing no atmospheric degradation.
wurtzite phase of CdS is in fact known [18] to undergo a size-induced structural phase transition to the zinc blende phase when the particle size is c2 – 3 nm. The XRD studies therefore indicate that only Sample 2 undergoes a drastic and rapid chemical degradation, while the other two samples do not appear to be affected by atmospheric exposure. We now examine this conclusion in the light of the optical micrographic studies of the surfaces of the films. Fig. 2(a) shows the surface features of the degraded sample (Sample 2). At lower magnification one clearly observes strongly faceted, bright green and
346
P. Taneja et al. / Materials Letters 54 (2002) 343–347
Fig. 2. Optical micrographs (true color) of the sputter-deposited nanocrystalline CdS thin films: (a) Sample 2, and (b) Sample 3.
light yellow crystalline grains, while it is clearer at higher magnification (not shown) that these crystallites show a stratified growth pattern. In sharp contrast, the surface of Sample 3 shows no visible degradation at any magnification (Fig. 2(b)). The scratch on the surface of this sample was made deliberately, to ensure sharp focusing of the surface of the film. This film is essentially smooth and featureless at optical magnifications. The micro-
graphic image of Sample 1 is very similar to that of Sample 3 and is not shown. A comparison of the X-ray and optical studies of Samples 1 and 3 clearly indicates that the chemical reactivity with ambient gases increases with a reduction in the average particle size of the nanocrystalline CdS thin films. This is only to be expected because of the larger specific surface area and the consequent presence of a relatively larger number of chemically
P. Taneja et al. / Materials Letters 54 (2002) 343–347
347
4. Conclusion We describe an rf-sputtering technique for producing nanocrystalline CdS thin films. When the particle size is very small (c2 nm), the nano-CdS undergoes chemical reaction with atmospheric components and gets rapidly degraded. Here we have reported a simple and viable technique for the long-term chemical passivation of such CdS films. This was accomplished by exposing the sputter-deposited nanocrystalline samples in-situ to pure hydrogen gas (or a hydrogen/nitrogen gas mixture) for about 20 h. No chemical degradation was detected on examining these films optically and by X-ray diffraction, even 6 months after the synthesis and passivation. The passivated nanocrystalline CdS films show strong, blue-shifted luminescence excitation and emission peaks at room temperature. Fig. 3. Photoluminescence spectra recorded at room temperature from Sample 3. The excitation spectrum (emission=437 nm) appears on the low-wavelength end while the emission spectrum (excitation=367 nm) appears at higher wavelengths. The emission from the bare Si substrate is shown by the dotted line at the bottom, on the same scale.
active surface sites in the smaller particles. A comparison of the data from Samples 2 and 3 shows that such surface chemical sites can be passivated by bonding with hydrogen gas, thereby reducing the reactivity of nanocrystalline CdS. Fig. 3 shows the photoluminescence spectrum from the hydrogen-passivated nanocrystalline CdS thin film (Sample 3) recorded at room temperature. The source and monochromator slits were both kept at 1 mm. The figure shows the excitation spectrum recorded at an emission wavelength of 437 nm, and the emission spectrum recorded with the excitation wavelength kept fixed at 367 nm (which corresponds to the peak in the excitation spectrum). The emission scan from the bare Si substrate is also shown. The peak excitation and the peak emission wavelengths are both appreciably blue-shifted with respect to the wavelength corresponding to the band gap energy in the bulk state (2.42 eV or 515 nm). The blue shift in the band-gap energy is related to quantum confinement effects in semiconductor nanoparticles.
References [1] L.E. Brus, Appl. Phys. A 53 (1991) 465. [2] A.P. Alivisatos, Science 271 (1996) 933. [3] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [4] B.O. Dabbousi, M.G. Bawendi, O. Onitsuka, App. Phys. Lett. 66 (1995) 1316. [5] L. Pavesi, L.D. Negro, C. Mazzoleni, G. Franzo, F. Priolo, Nature 408 (2000) 440. [6] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [7] Y. Wang, N. Herron, J. Phys. Chem. 91 (1987) 257. [8] B.G. Potter, J.H. Simmons, Phys. Rev. B 37 (1988) 10838. [9] H. Mathieu, T. Richard, J. Allegre, P. Lefebvre, G. Arnaud, J. Appl. Phys. 77 (1995) 287. [10] N. Chestnoy, T.D. Harris, R. Hull, L.E. Brus, J. Phys Chem. 90 (1986) 3393. [11] R. Rossetti, R. Hull, J.M. Gibson, L.E. Brus, J. Chem. Phys. 82 (1985) 552. [12] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [13] D.J. Norris, A. Sacra, C.B. Murray, M.G. Bawendi, Phys. Rev. Lett. 72 (1994) 2612. [14] L. Spanhel, M. Haase, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 5649. [15] A.R. Kortan, R. Hull, R.L. Opila, M.G. Bawendi, M.L. Steigerwald, P.J. Carroll, L.E. Brus, J. Am. Chem. Soc. 112 (1990) 1327. [16] R. Chandra, P. Taneja, P. Ayyub, G.K. Dey, S.K. Kulshreshtha, Nanostruct. Mater. 11 (1999) 1171. [17] P. Ayyub, R. Chandra, P. Taneja, A.K. Sharma, R. Pinto, Appl. Phys. A 73 (2001) 67. [18] R. Banerjee, R. Jayakrishnan, P. Ayyub, J. Phys.: Condens. Matter 12 (2000) 10647.