J. Phys. Chem. 1995, 99, 16442-16448
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Quantum Size Effects in Chemically Deposited, Nanocrystalline Lead Selenide Films Sasha Gorer: Ana Albu-Yaron,' and Gary Hades*$' Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel, and ARO, The Volcani Centre, POB 6, Bet Dagan 50250, Israel Received: July 12, 1995@
Quantum size effects have been studied in chemical solution deposited films of nanocrystalline PbSe. Films have been deposited containing crystals of varying size and size distribution by varying the deposition parameters, in particular the nature of the complexing agent, the complex-to-lead ion ratio, the film thickness, and the deposition temperature. Crystal size and size distribution were measured by transmission electron microscopy and correlated with the increase in effective optical bandgap estimated from the optical absorption spectra of the films. Postdeposition treatments (annealing and treatment in hydroxide solutions) were used to induce controlled crystal growth and the bandgaps of the treated films were correlated with the crystallite size. The experimental increases in bandgap were compared with theoretical models for increase in bandgap as a function of nanocrystallite size. X-ray photoelectron spectroscopy showed that the PbSe in the highly quantized films was intrinsic, with the Fermi level close to the center of the effective bandgap.
Introduction Semiconductor nanocrystals, and more specifically semiconductor quantum dots, has become an increasingly studied and interdisciplinary subject over the past decade. The interest in this field stems largely from the size-dependent energy level structure, and hence optoelectronicproperties, of these materials. The research on quantum dots can be divided into two areas depending on the preparation technique: reducing the size of a larger structure (such as lithographic patteming of superlattices) and growing the quantum dots directly. Most work on this latter area has been carried out on dispersed nanocrystals in a liquid or glassy matrix with some studies on other matrixes such as polymers.' While quantum size effects have been reported on ultrathin ( < 5 nm) semiconductor it is only recently that such effects have been seen in thick films of aggregated nanocry~tals.~-~ Such films possess several advantages over dispersed nanocrystals, in particular the ability to control the potential of such films on electrically conducting substrates and also to pass current through them, allowing the study of charge transfer in these materials. Most of these studies of quantum size effects have concentrated on 11-VI and, to a much lesser extent, oxide semiconductors. Of the remaining semiconductors, PbS has probably been the most investigated. Relatively few studies have been reported on lead selenide.2x5-" However, PbSe should show very large quantum effects due predominantly to the very low values of electron and hole effective masses (average electron, hole, and reduced effective masses are 0.047, 0.041, and 0.022, respectively'*), since the blue spectral shift in the absorption spectra as a function of crystal size is inversely proportional to the reduced effective mass from the simple particle-in-a-box model of charge localization in the nanocrystals. For a crystallite size less than the Bohr diameter (ca. 80 nm for PbSe), an increase in the effective bandgap due to size quantization should begin to be apparent. As far back as 1960, blue shifts in the photoconductivity spectral response of evaporated PbTe and PbSe films were attributed to small crystal size, although no connection to size quantization was made at that time.I3 Weizmann Institute of Science. The Volcani Centre. @Abstractpublished in Advance ACS Abstracts, October 1, 1995 +
Nedeljkovic et al. demonstrated the shift in both optical spectra and in reduction potential of PbSe colloids in water and in acetonitrile.I0 Chang et al. showed large blue shifts of the absorption spectra of PbSe nanocrystals precipitated on the surface of surfactant vesicles." In a series of papers, a Romanian group studied photoconductivity in chemically deposited (CD) PbSe films5,'.* and found blue shifts in the photoconductivity spectra which they attributed to quantum size effects. Specifically, they measured crystal sizes of 40-60 nm in the as-deposited films and resulting blue shifts of 0.07 eV.8 We have shown preliminary results of large blue shifts in the optical transmission spectra of CD PbSe films.6 We have been investigating CD PbSe films with particular emphasis on control of the crystallite size in the films and have succeeded in depositing films with crystal sizes ranging from ca. 3 nm up to micron^.'^ In this paper, we describe the wide range of optical properties obtained from these films resulting from the size quantization in the individual nanocrystals comprising the films.
Experimental Section Film Deposition. Three different complexing agents were used to deposit the films: trisodium citrate (TSC), potassium nitrilotriacetate (NTA), and potassium hydroxide (KOH). For the depositions from the TSC bath, the following stock solutions were made up (using deionized water): 0.5 M lead acetate, 1 M TSC, and 0.2 M sodium selenosulfate (Na2SeSO3) in excess Na2S03, prepared by stirring 0.2 M Se with 0.5 M Na2S03 at ca. 70 "C for several hours. The deposition solution was prepared by diluting the 0.5 M lead acetate solution with water, then adding the TSC solution, and adjusting the pH with KOH to 10. Finally, NazSeSO3 was added, giving a pH 5 10.8, which was then adjusted to exactly 10.8. The composition of the final solution was 60 mM Pb2+, 50 mM NazSeSO3, and 160-320 mM TSC. For the depositions from the NTA bath, the same stock solutions were used as above except that 0.7 M K3NTA was substituted for the TSC. The deposition solution was prepared as for PbSe from TSC complex to give a final composition of 60 mM Pb2+,50 mM Na2SeSO3, and 60-70 mM NTA at a pH of 10.8-11.0. For the films from KOH complex, concentrated KOH solutions (1 M or greater) were used as the complex stock
0022-365419512099-16442$09.0010 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 44, 1995 16443
Nanocrystalline Lead Selenide Films solution. The deposition solution was prepared as before to give a final composition of 60 mM Pb2+, 50 mM NazSeSO3, and 0.6-4.3 M KOH at a final pH > 13. The lead acetate stock solution was added to the KOH; if the reverse procedure was used (addition of KOH to the Pb solution), a precipitate formed that did not readily dissolve unless a large excess of KOH was added. By adding the Pb solution to KOH, a large excess of KOH was maintained (up to a certain concentration of Pb2+) which prevented precipitation of the hydrated oxide. Microscope glass slides (40 x 15 x 1 mm) were used as substrates. They were cleaned in sulfochromic acid and well rinsed in deionized water prior to the deposition. The substrate was placed at an angle (typically ca. 20" off normal) in the solution, which was then placed in thermostat at the required temperature and kept in the dark. The deposition time depended on the conditions and the required thickness (20-80 nm for most films). It was typically several hours but could range from tens of minutes to days. The film on the upper side of the substrate was removed with a cotton swab moistened with nitric acid:water (1:4). The film deposited on the lower side of the substrate was retained for the various measurements. It was strongly adherent. While most measurements were carried out on as-deposited samples, for some experiments, the samples were annealed in air or treated in KOH solutions of different concentrations. A p-type PbSe single crystal was used as a reference for XPS measurements. Optical Characterization. The optical transmittance ( r ) and reflectance ( R )spectra of the PbSe films were recorded using a Cary 17D infrared-visible spectrophotometer in the 400-3000 nm range. Spectral data are given as absorption coefficient (a) vs photon energy, as these are the most relevant parameters for the purposes of this paper. The absorption coefficients were calculated from the transmittance spectra and film thickness. The transmittance was corrected for specular reflectance loss, which was fairly high for many of the films. This was done by using the correction T = Tmeas/(l- R), where T is the real transmission and Tmeas the measured value. The correction often used based on (1 - R)2 in the denominator was not suitable here, due mainly to the much lower expected reflectance from the PbSe/glass interface compared with the airPbSe interface, to the very thin nature of many of the films and to the fact that the product of absorption coefficient and film thickness, ad, was relatively small for many of the measurements. X-ray Powder Diffraction. Powder X-ray diffraction (XRD) spectra of films deposited on glass or Ti were recorded on a Rigaku RU-200B Rotaflex diffractometer operating in the Bragg configuration using CuKa radiation. The accelerating voltage was set at 56 kV with a 170 mA flux. Scatter and diffraction slits of 0.5" and a 0.3 mm collection slit were used. Transmission Electron Microscopy. A Philips EM400T electron microscope operating at 120 kV was used for transmission electron microscopy (TEM). Imaging was carried out in bright field. Samples were prepared by removing the films from the substrates onto copper grids (300 mesh) by light etching in dilute (1-3%) HF. X-ray photoelectron spectroscopy (XPS) was used to measure the Fermi level position with respect to the valence band edge and the valence band ~tructure.'~ The spectra were obtained on an "AXIS HS Kratos" XPS/Auger surface analysis system using a magnesium K a monochromated photoline. All samples were grounded. Results and Discussion Before describing the optical properties of the PbSe films, the important mechanisms of nucleation and growth of the PbSe crystallitesand the exhibited microstructure of the various films, described by us recently,I4 will be outlined.
TABLE 1: Crystal Sizes, Measured by TEM and XRD, of Various Films Deposited from the Three Complexants under Different Conditions"
complex TSC
preparation condition (comp1ex:Pb ratio, temp, thickness) 2.67 (LC), 0 "C, 20 nm 2.67 (LC), 0 "C, 60 nm 2.67 (LC),60 "C, 20 nm 2.67 (LC), 60 "C, 60 nm 5.33 (HC),0 "C, 70-80 nm 5.33 ( H C ) , 60 "C, 70-80 nm
NTA
1.00 (LC), 0 "C, 30 nm 1.00 (LC),0 "C, 70 nm 1 .00 (LC), 60 O C , 30 nm
1.OO (LC), 60 O C , 70 nm
size from
TEM (nm) 3.5-4.0 3.5-4.0 and 6.0- 12.0 4.0-4.5 4.0-4.5 and 6.0-12.0 8.0-15.0 8.0-15.0 3.5-7.0 3.5-7.0 and 9.0-25.0 5.5-7.0 5.5-7.0 and 15.0-35.0
size from XRD (nm)
___ 6.5
--_ 7.2 10.0 15.0
--10.0
___ 30.0
_-_ 3.5-4.0 c5.0 3.5-4.0 30.0 3.5-4.0 and 300-500 --5.0 (LC), 60 "C, 30 nm 10.0-20.0 12.0 10.0-20.0 5.0 (LC),60 "C, 70-80 nm 36.0 (HC), 60 "C, 80-100 nm 1000 (SEM) a The large crystal sizes ('ca. 50 nm) could not be measured by XRD due to instrument broadening. Broken lines represent XRD data which showed no peaks. Adapted from ref 14. KOH
10.0 (LC),0 "C, 30 nm 10.0 (LC),0 "C, 70-80 nm 0 "C, 80-100 nm 72.0 (HC),
Three complexing agents were used for Pb2+ ions: TSC, NTA, and OH- (in order of increasing complexing strength). Several factors determined the crystal size. Most important was whether a colloidal basic lead carbonate or hydroxide was present in the solution (depending on the amount and strength of the complexant). The presence of appreciable amounts of such a colloid resulted in a hydroxide-mediated mechanism and smaller PbSe crystal size (designated as LC, low complex); in the absence of such a colloid (higher complexant concentration [designated by HC,high complex] or stronger complexant), an ion-by-ion mechanism resulted in larger crystal size.I6 Other factors of importance were temperature of deposition (particularly for the OH--complex films) and film thickness. For TSC and NTA films, while very thin films (ca. 20 nm) were homogeneous (in terms of crystallite size), substantially thickner films exhibited a two-domain structure of small and larger crystals where most of the film was composed of the small crystals, but micron-sized domains of exclusively larger crystals occurred, usually comprising not more than 10% of the total area of the films. The LC films-particularly the thin TSC ones-were often comprised of varying amounts of an amorphous PbSe matrix which surrounded the individual crystallites. Typical crystal sizes of the various films are given in Table 1, which is a condensed form of Table 2 in ref 14. Figure 1 presents TEM images of a ca. 60 nm thick PbSe film deposited from a LC TSC bath at 0 "C. showing in Figure 1A small crystallites (>go% of the film), and in Figure 1B crystallites from a large crystal domain in the film. We do not show a separate micrograph of a HC film as the crystallites in Figure 1B are similar, if a little smaller on average, to those in the HC films. There is only a weak dependence of crystallite size on deposition temperature for these films. More detailed TEM and XRD data of these and the other films described in this paper are shown in ref 14. Figure 2 gives the absorption coefficient (a)of various TSC films as a function of light energy (energy is used rather than
16444 J. Phys. Chem., Vol. 99, No. 44, 1995
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Figure 1. TEM images of ca. 60 nm thick PbSe films deposited on glass substrates from TSC solution at 0 "C under LC conditions (small crystal (A) and large crystal (B) domain).
0.5
1
I .5
2
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ENERGY (eV)
Figure 2. Plots of a vs hv for PbSe films deposited from TSC under various conditions. An evaporated film (50 nm thick, crystal size ca. 40 nm) is shown here as a "bulk" (Le., virtually no size quantization) standard. (1) 60 "C,HC,70 nm thickness; (2) 60 "C C, LC, 60 nm; (3)0 "C,HC,70 nm; (4) 0 "C, LC,60 nm; (5) 60 "C,LC,20 nm; (6) 0 "C,LC, 20 nm. wavelength as it is more relevant to the quantum effects we describe here). Also shown is a 50 nm thick evaporated (onto a room-temperature substrate) PbSe film, with an average crystal
Figure 3. TEM images of 70 nm thick PbSe films deposited on glass from NTA solution at 0 "C under LC conditions. Small crystal (A) and large crystal (B) domains. size of ca. 40 nm, as a reference for a (virtually) nonquantized film. It is clear that a wide range of different optical properties can be obtained with values of effective Eg ranging from 0.55 to 1.55 eV. The clear inflections in samples 2 and 4 correlate with the two different crystal sizes in these films-small crystallites of ca. 4 nm and a small concentration of larger ones of typically 6- 12 nm; the low-energy tail comes from the larger crystallites, while the high-energy absorption comes predominantly from the small crystallites, with some contribution from the strong high-energy absorption of the (low concentration of) larger ones. This inflection is missing in samples 5 and 6 (very thin films) where only small crystallites occur. The smaller inflection seen in the spectra of the HC samples (1 and 3), as well as in that of the evaporated film, is characteristic of the absorption spectra of bulk (i.e., nonquantized) PbSe. "EM images of a typical NTA film are shown in Figure 3A (LC,0 OC, ca. 70 nm thick) and 3B (large crystal domain of the same film which is not seen in thin LC films). The main differences compared to the TSC films are a wider size distribution of the small crystallites (3.5-7 nm) and larger crystals with a wide size distribution in the large domain regions.
Nanocrystalline Lead Selenide Films
0°C LC 7Onm
....I
..;
-.-.-60°C LC 30nm ..I
J. Phys. Chem., Vol. 99, No. 44, 1995 16445
1
/-;
::
i
dl t: w
0
L,
I
ENERGY- (ev)
Figure 4. Plots of a vs hv for PbSe films deposited from NTA under various conditions. The same evaporated film as in Figure 2 is used as a bulk standard.
HC NTA films are not considered in this study as they are invariably highly scattering. The a vs photon energy plots of four NTA films, together with the evaporatedfilm standard, are shown in Figure 4. While they are rather similar to corresponding TSC films, there are differences which can be correlated with the different size characteristics of these films. Films 3 and 4 (30 nm thick and composed almost entirely of small crystallites) show a distinct curvature in the high-energy region compared with the almost linear TSC thin films. This can be explained by the larger size distribution in the NTA films, which results in a less-defined and lower effective value for Eg due to smearing out of the absorption edge. They exhibit only a weak absorption in the low-energy region resulting from the low concentration of large crystal domains (somewhat higher in the high temperature film, 3), which cannot be easily distinguished from absorption by the larger (7 nm) crystals in the small crystal regions. The thicker (70 nm) LC films in Figure 4 show a much more pronounced low-energy tail due to the large-crystal domains. This tail extends to lower energy than the corresponding TSC films due to the larger crystallite sizes in the large-crystal domains (ca. 10-25 nm) compared with those in the TSC films (6-12 nm). The KOH films differ from the other films in that the twodomain structure was absent and the effect of deposition temperature was much more marked in the LC films. TEM images of a 0 "C HC film and both LC and HC films at 60 "C are shown in Figure 5A-C, respectively). The coexistence of large, cubic crystals (ca. 400 nm) covered with ca. 4 nm crystallites is particularly noticeable in the 0 "C HC film. The micrographs of the 0 "C LC films were very similar to those of the equivalent TSC films (Figure 1A) but without the large crystal domains and with less amorphous matrix and are not shown separately here. The correspondingabsorption spectra of similar films to those in Figure 5 are shown in Figure 6. The absence of the inflection due to the two-domain structure in the LC films is evident. Also, unlike the TSC and NTA films, there is no appreciable difference in the spectra of thin and thick LC films, again due to the absence of the two-domain structure. There is a large difference between the 0 "C LC and 60 "C LC films due to their different crystal sizes (4 and ca. 12 nm, respectively). The presence of a two-domain inflection is seen in the 0 "C HC film, corresponding to the two crystal sizes occurring in these films (Figure 5A) but is absent in the 60 "C HC film (Figure 5C), which is composed only of large (ca. 1 pm) crystals and which gives a spectrum essentially identical to that of the nonquantized evaporated film.
Figure 5. TEM images of PbSe deposited from KOH solution at 0 "C, HC conditions (A); 60 "C LC conditions (B); 60 "C HC conditions
0. The large crystal sizes of the HC KOH films can be explained by the effect of strong alkaline solutions on nanocrystalline films. This effect has been reported for nanocrystal-
16446 J. Phys. Chem., Vol. 99, No. 44, 1995
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2.5
Figure 6. Plots of a vs hv for PbSe films deposited from KOH under various conditions.
U
O.7M KOH
I . . . _
f X
Y
2
-
0.5
-0.15M KOH
I
1.5
2
2.5
ENERGY (eV)
Figure 7. Plots of a vs hv for PbSe films deposited from TSC (0 OC, LC, 70 nm thickness) before (as deposited) and after different KOH treatments.
line CdSe films'* and is even more apparent for the PbSe films. Figure 7 demonstrates the effect of several different concentrations of KOH solutions (10 min immersion at room temperature) on the absorption spectrum of a LC TSC film. Little effect is seen until a KOH concentration of 0.15 M is used. At this concentration, a small red shift is seen on both the large crystal and small crystal domain absorptions. This shift becomes larger with increasing concentration of KOH until the bulk spectrum of PbSe is reached at 0.9 M KOH. This shift correlates with increasing crystallite size as the KOH concentration is increased (Figure 8). This grain growth may be due to selective dissolution and redeposition of the aggregated crystallites. However, while we only speculate on the mechanism here (this phenomenon will be the subject 6f a separate study), the important point is that the high concentrations of KOH present in the HC KOH films can phenomenologically explain the large crystal size obtained. A more obvious method of postdeposition growth of crystallite size is to heat the films. Figure 9 gives the absorption spectrum of a thick (ca. 70 nm) 0 "C LC TSC film and the spectra after annealing in air at increasingly higher temperatures. Figures 10 and 11 give the corresponding TEM and XRD data, respectively. At the lowest temperature (150 "C), Figure 9 shows a fairly large red shift of the small-crystal part of the spectrum but has virtually no effect on the largecrystal domains. This is expected, as such a low temperature should affect only very small crystals of the PbSe. After annealing, there is a wide distribution of crystal sizes from the original size of 3.5-4.0 nm (Figure 10A) up to at least 10 nm (Figure lOB), which explains this red shift. At 300 "C, there is a large increase in
Figure 8. TEM images of PbSe films deposited from TSC (0 "C, LC, 60 nm thickness) after 10 min immersion in 0.7 M (A) and 0.9 M (B) KOH solutions.
U
0.5
1
1.5 ENERGY (eV)
2
2.5
Figure 9. Plots of a vs hv for PbSe films (ca. 60 nm thickness, 0 OC, LC TSC) as-deposited and after annealing in air for 30 min at 150, 300, and 400 OC. The samples were cut from one larger sample and each one annealed at a different temperature. The evaporated standard film is also shown.
crystal size to typically 25-35 nm. After annealing at 400 "C, when the typical crystal size is 70 nm, the bulk Eg of 0.28 is expected. While we could not measure the spectra below 0.5 eV, extrapolation of the 400 "C film to lower energy is entirely consistent with this bulk value, while the 300 "C film gives an estimated value for Eg of ca. 0.4 eV-a relatively small shift
J. Phys. Chem., Vol. 99, No. 44, 1995 16447
Nanocrystalline Lead Selenide Films
Figure 10. TEM images of PbSe film deposited from TSC at 0 "C (ca. 7 0 nm thickness, LC conditions) before annealing (A) and annealed at 150 (B),300 (C), and 400 "C (D). Scale bar = 70 nm.
7 ---..1-1-
m*=0.022
- - - m*=0.040 -exp.
data
" 10 0
0
0 0
20
-3
20
30
40
Nanocrgstal diameter (nm)
~ o x o " x
60
26
I
60
'
j0
80
(deg)
Figure 11. XRD spectra of the PbSe film from Figure 10 before annealing (A) and anealed at 150 (B), 300 (C) and 400 "C (D). All peaks correspond to PbSe with the following exceptions. Those marked 6PbCOy3Pbwith x correspond to elemental Pb; 0, Pb504S04; (0H)yPbO (from ref 14). The small doublet peak in the 400 "C spectrum at 28 = ca. 47" was unidentified.
+,
from the bulk value. The XRD data (Figure 11) also show the presence of small amounts of other phases (Pb3(C03)2(OH)2, Pb, Pb~04S04)in the annealed films. It appears that these impurity phases do not influence the spectra to any large extent. A comment on the amorphous matrix which is present in the LC films-particularly the thin TSC ones. We do not see any obvious signs of absorption by this matrix. Such absorption should be particularly noticeable in the thin 0 "C LC TSC film (Figure 2, graph 6). The fact that, at high photon energies, a for this film is smaller than for the equivalent KOH film in Figure 6, could be interpreted as arising from a lower concentration of crystalline PbSe in the former (ais calculated from film thickness assuming a homogeneous film; this assumption is incorrect for the above TSC film). In this case, absorption by the matrix in the region of interest would be small compared with that of the crystalline material and our values of a for the thin TSC films will be somewhat underestimated. The estimation of Eg from these plots will not, however, be appreciably a1tered. Having described a large variety of PbSe films with different crystallite sizes (and size distributions) and having correlated these crystal sizes with absorption spectra, we now compare the results with theoretical models for the effect of crystal size on Eg for PbSe. The parabolic band model, which gives only a fair fit to experimental data for medium-to-larger nanocrystals
Figure 12. Calculations of PbSe bandgap as a function of crystal size using the hyperbolic band approximation with averaged reduced effective mass values of 0.022 (literature value) and 0.04, and experimentally measured values. Since in most cases, the uncertainty in the crystal size is greater than that in the bandgap (due to the size distribution), the "error" bars are given as horizontal lines. Some of the experimental values are from experiments and samples not shown in this work.
of 11-VI semiconductors (it increasingly overestimatesthe blue shift for small nanocrystals) is even less applicable to PbSe where the bands are far from parab01ic.I~A better approximation has been used for PbS based on hyperbolic bands,20 and we use this model, with correction for the electron-hole Coulomb attraction, with which to compare our results. Figure 12 shows the bandgap calculated for PbSe as a function of (assumed spherical) crystal diameter using this model, together with experimental values taken from our results. Values of Eg less than 0.5 eV were obtained by extrapolation of the a vs hv plots to zero a. Two averaged reduced effective masses (m*) are shown. For the literature value (O.022m&I2 while the theoretical model does give a reasonable prediction of Eg, it does overestimate the value for small crystallite sizes, in common with all effective mass-based models. The theoretical curve based on an arbitrary value for m* of 0.04, which gives an overall better fit for the smaller crystallite sizes, is also shown in Figure 12. Finally, some XPS data comparing Fermi level positions and valence band structure for LC and HC TSC films and for singlecrystal PbSe are shown in Figure 13. In the LC film (E, ca. 1.3- 1.4 eV from Figure 2), the Fermi level, EF is located 0.65 eV above the valence band, Le., in the middle of the gap. This means that the PbSe is intrinsic, as might be expected for such small crystals where even a single active dopant atom would cause a drastic change in the crystal properties and position of
16448 J. Phys. Chem., Vol. 99, No. 44, 1995
Gorer et al. of size quantization in the individual crystallites. A large variation in crystallite size (from a few nanometers up to micrometers) could be realized both by control of the deposition parameters and by postdeposition treatments. This was paralleled by a variation in effective bandgap of the films from the bulk value of 0.27 eV up to greater than 1.5 eV. We fit the variation in bandgap as a function of crystal size to a theoretical (hyperbolic band) model, with a phenomenologically chosen effective mass parameter for the smaller crystallite sizes. Using X-ray photoelectron spectroscopy,the Fermi levels of the highly quantized films were found to be situated close to the center of the effective bandgap.
References and Notes (1) For a comprehensive review see: Yoffe, A. D. Adu. Phys. 1993, 42, 173.
Figure 13. XPS valence band spectra obtained from LC sample (TSC, 60 "C,ca. 4 nm crystallites); HC sample (TSC, 60 "C, 8-15 nm crystallites), and single-crystal PbSe.
EF. For the larger HC crystals (Eg, ca. 0.55 eV from Figure 2), EF is ca. 0.38 eV above the valence band, suggesting n-type behavior. However, bearing in mind that a difference of 0.1 eV (the typical uncertainty of this method) in such a low value would change this interpretation considerably, we cannot place EF with any degree of accuracy, expect to state that it is not very close to the valence band. For the single-crystal bulk sample, EFis very close to the valence band (within the accuracy of the technique). This is consistent with the low Eg(0.27 eV), p-type nature of this sample. The difference between the various samples is a direct and independent measure of the size quantization occurring in these films. The change in the density of states of the valence band from bulk or close to bulk (single crystal and HC film) to highly size-quantized (LCfilm) PbSe is also seen, at least in a general way, by increasing structure in the valence band for the highly quantized film compared with the large crystal (up to 15 nm) and bulk samples, which are similar to each other (the HC sample gave a much stronger signal than either of the other two samples, which explains the lower noise level and therefore smoother spectrum). In conclusion, we have shown how the optical absorption spectra of chemically deposited PbSe films vary with the size and size distribution of the crystallites in the films as a result
(2) Salashchenko, N. N.; Filatov, 0. N. Sou. Phys. Semicond. 1979, 13, 1017. (3) Papavassiliou, G. C. J. Solid State Chem. 1981, 40, 330. (4) Hodes, G.; Albu-Yaron, A,; Decker, F.; Motisuke, P. Phys. Rev. B 1987, 36, 4215. ( 5 ) Biro, L. P.; Darabont, Al.; Fitori, P. Europhys. Lett. 1987, 4, 691. (6) Hodes, G.; Albu-Yaron, A. Proc. Electrochem. SOC.1988,88-14, 298. (7) Biro, L. P.; Candea, R. M.; Borodi, G.; Darabont, AI.; Fitori, P.; Bratu, I.; Dadarlat, D. Thin Solid Films 1988, 165, 303. (8) Dadarlat, D.; Candea, R. M.; Turcu, R.; Biro, L. P.; Zasavitskii, I. I.; Valeiko, M. V.; Shotov, A. P. Phys. Status Solidi A 1988, 108, 637. (9) Hodes, G. Isr. J . Chem. 1993, 33, 95. (IO) Nedeljkovic, J. M.; Nenadovic, M. T.; Micic, 0. I.; Nozik, A. J. J . Phys. Chem. 1986, 90, 12. (11) Chang, A.-C.; Pfeiffer, W .F.; Guillaume, B.; Baral, S.; Fendler, J. H. J. Phys. Chem. 1990, 94, 4284. (12) Dalven, R. Infrared Phys. 1969, 9, 141. (13) Lawson, W. D.; Smith, F. A.; Young, A. S. J . Electrochem. SOC. 1960, 107, 206. (14) Gorer, S.; Albu-Yaron, A,; Hodes, G. Chem. Mater. 1995, 7, 1243. (15) The use of the "bulk' terms, such as bandgap (E,) and valence band, are used here for ease of reading. It should be kept in mind that in the quantized material described here what is meant by these terms is first optical transition and upper filled level. (16) Gorer, S.; Hodes, G. .I. Phys. Chem. 1994, 98, 5338. (17) Bauer, G.; Krenn, H. In Handbook of Optical Constants ofSolids; Palik, E. D., Ed.; 1985; p 517. (18) Hodes, G.; Howell, I. D. J.; Peter, L. M. In Photochemical And P hotoelectrochemical Conuersion and Storage of Solar Energy ( Proc. IPS9); Tian, Z . W., Cao, Y., Eds.; Intemational Academic: Publishers, Beijing, 1993; p 331. (19) Dubrovskaya, I. N.; Efimova, B. A.; Nensberg, E. D. Sou. Phys. Semicond. 1968, 2. 436. (20) Wang, Y . ; Suna, A,; Mahler, W.; Kasowski, R. J . Chem. Phys. 1987, 87, 7315. Jp951959P
