© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 32, AUGUST 16, 2001
LETTERS Pressure Tuning Optical Absorption Spectroscopy of Erbium-Doped Silicon Nanocrystals John V. St. John and Jeffery L. Coffer* Department of Chemistry, Texas Christian UniVersity, Ft. Worth, Texas 76129 ReceiVed: December 31, 2000; In Final Form: March 21, 2001
High-pressure optical absorption spectroscopic measurements of both erbium-doped and undoped Si nanoparticles have been carried out in a diamond anvil cell up to pressures of 180 kbar. The emphasis here is with respect to (a) the effect of particle size on the pressure dependence of the band gap as well as (b) indirect examination of the structural impact of the erbium dopant on the pressure-induced phase transition(s). It is found that in terms of electronic structure these Er-doped Si nanocrystals act very much like indirect gap silicon, with an observed band gap pressure dependence of -1.4 × 10-6 eV/bar. Measurements of the optical spectra in terms of integrated area as a function of pressure of these doped nanoparticles reveal that the first-order phase transition must lie above 180 kbar, substantially elevated from the bulk value of 120 kbar. Thus, doped nanocrystals of this dimension maintain a significant elevation in the phase transition pressure (known in homogeneous Si nanocrystals relative to bulk crystalline Si), but the Er dopant does not introduce the type of structural defects that would lower the energy barrier to such a transformation.
The behavior of crystalline Si under high pressure has been the subject of intense experimental and theoretical interest for well over 30 years.1-14 Studies via optical absorption,1,2,7 electrical resistivity,3,6 and X-ray diffraction techniques4,5,8-13 have provided useful information concerning both structure and bonding for this technologically crucial material. In terms of band structure, it is now well established that the experimental (dE/dP)T value for cubic Si is roughly -1.4 × 10-6 eV/bar, a consensus emerging from both optical absorption and corrected resistivity measurements.7 In the bulk, this observed red shift is attributed to displacement of conduction band minima along the 〈100〉 direction to lower energy with increasing pressure (relative to the valence band maxima at γ), thereby effectively reducing the band gap.1 Structurally, a number of interesting pressure-induced phase transitions have been reported in crystalline Si, most thoroughly the diamond cubic to β-tin transformation that occurs at ∼12 GPa.4,5,8-14 * To whom correspondence should be addressed. E-mail:
[email protected].
Recent experiments concerning crystalline Si of nanometer dimensions (quantum dots) capped with oxide have demonstrated a remarkable elevation in the first-order phase tranistion from 12 to above 22 GPa.12 Such studies did not address, however, the pressure effects on the band gap in nanophase Si-containing structures (as extracted from optical absorption measurements). The latter is important in analyzing fundamental issues in the electronic structure of Si quantum dots, such as the question of “bulklike” versus “surface-like” character in the band edge wave functions of these materials.15-17 We have recently reported the doping of discrete Si nanoparticles with optically active erbium centers and characterized these nanostructures by transmission electron microscopy (TEM), selected area electron diffraction, X-ray energy dispersive spectroscopy, photoluminescence (PL), and UV-visible absorption spectroscopies.18 In this report we address the effect of pressure on the optical absorption edge of both Er-doped and undoped Si nanocrystallites, with an emphasis on (a) the effect of particle size on the pressure dependence of the band
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Figure 1. Effect of pressure on the visible absorption spectrum of Er-Doped Si nanocrystals of 3.3 nm average diameter.
gap as well as (b) indirect examination of the structural impact of the erbium dopant on the pressure-induced phase transition(s). Erbium-doped as well as undoped Si nanocrystals were formed from the pyrolysis of disilane in a He atmosphere at 1000 °C according to a previously published procedure.18 For the rare earth-doped nanocrystals, Er3+ was introduced into the He stream in the form of the β-diketonate complex Er(tmhd)3. Silicon nanocrystals with typical erbium concentrations on the order of 2% were extracted, washed, and purified according to the procedure originally described in ref 18. Based on the preparative method, the surface of both doped and undoped nanoparticles are oxide-capped. Pressure is applied to these samples (of ∼12.5 µm thickness) with the use of a diamond anvil cell (DAC) of Merrill-Bassett design, with ethylene glycol used as a pressure-transmitting medium. Alternatively, experiments employing methanol:ethanol for this purpose yielded comparable results. The type II diamonds employed in our experiments have an upper pressure limit of 180 kbar. Pressure calibration was achieved using measurements of the R1 and R2 fluorescence lines of ruby,19 measured in this case by the use of a Nikon Optiphot fluorescence microscope interfaced to a Scientific Measurements System 0.5 m monochromator with a Santa Barbara Instruments Group CCD. UV-visible spectra in the range 400-800 nm were taken using an Ocean Optics S-2000 fiber optic spectrometer at a resolution of (0.2 nm. A 250 W tungsten halogen lamp was used as the source, which was coupled to the diamond anvil cell using a single core silica fiber; the transmitted light was coupled to the detector using another single core fiber. The optical absorption spectra of Er3+-doped Si nanocrystallites (average diameter 3.3 nm) in the UV-visible region at atmospheric pressure show a broad absorption tail with an onset in absorption near 630 nm (Figure 1). An indirect gap semiconductor such as Si typically exhibits a broad absorption tail in the visible with an abrupt increase of optical density in the UV. The weak tail from 370 to 700 nm is responsible for the yellow color of the nanocrystals and is ascribed to the weak indirect absorption at the indirect gap of Si.20 The stronger absorption at 370 nm and below is due to the higher energy direct gap. This type of absorption spectrum is observed in all samples, both doped and undoped. In general, the onset of absorption of the indirect gap shifts to lower energy as pressure is applied to a given sample in the diamond anvil cell (Figure 1). Fitting this absorption data as a function of the square root of absorptivity versus energy (eV) yields a slightly curved vertical line for a given pressure measurement.21 Figure 2
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Figure 2. Pressure effects on the absorption edge of Er-doped Si nanocrystals (mean d ) 3.3 nm), represented in terms of the square root of absorptivity as a function of energy. Absorption edge values at atmospheric pressure are confirmed by examining the excitation spectrum of a visibly emitting undoped Si nanocrystal sample of comparable size.
Figure 3. Observed dependence of energy gap versus pressure for Er-doped Si nanoparticles of 3.3 nm mean diameter.
TABLE 1: dE/dP Values for the Si Nanocrystals Evaluated in This Study nanocrystal
dE/dP (eV/bar)
Er-doped Si 3.3 nm Er-doped Si 5.9 nm undoped Si 9.3 nm undoped Si 12.3 nm
-1.3 × 10-6 -1.4 × 10-6 -1.4 × 10-6 -1.3 × 10-6
illustrates the shift of the data plotted in this manner for an Er-doped Si nanocrystal sample for several pressure values from 14 to 180 kbar. It is clear that the overall effect of increasing pressure shifts the onset of absorption to lower energy. Extrapolation of each value to the x-axis and subsequently plotting this energy value as a function of pressure yields a straight line, exemplified in Figure 3 with a slope of -1.4 × 10-6 eV/bar for this 3.3 nm Er-doped Si sample. Such a value correlates well with the established literature values of (dE/ dP)T for bulk Si with the diamond cubic structure.7 We have subsequently measured the effect of pressure on the optical absorption edge of several different sample types for Si nanocrystals both with and without Er. The results are summarized in Table 1. The values for the extracted shift of the band gap with pressure for our Si nanostructures (both doped and undoped) are consistent with the computational studies of Zunger and co-workers.15-17 Their evaluation of pressure shifts for a series of Si quantum wires via pseudopotential methods reveal values that do increase slightly as the dimensions of the wire increase (from 4 × 4 f 6 × 6f 8 × 8f 10 × 10), but that all computed values nevertheless fall within the range of experimentally observed values for bulk Si.17 This is not surprising, since in these Si nanostructures the conduction band
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minima (CBM) are composed mainly of bulklike states; for example, in an 8 × 8 wire the CBM is 78% of the two lowest energy bulk conduction bands.15 Thus, taken in concert, it would appear that the conduction band levels in the case of the Erdoped Si nanocrystals evaluated here are predominantly bulklike in character as well. The second piece of information that can be extracted from these experiments is with respect to phase transitions. This is most conveniently achieved by evaluating changes in the integrated optical density in the visible portion of the spectrum as a function of pressure. For bulk crystalline Si, a sharp increase in optical density is anticipated from the diamond cubic to β-tin phase change near 120 kbar,7 a consequence of the change in the dielectric response of the metallic β-tin phase. However, silicon nanocrystallites show increases in optical density on the order of several hundred percent at much higher phase transition pressures of approximately 220 kbar.12 This elevation in phase transition pressure can be qualitatively understood in terms of terms of an increase in the surface energetics of the highpressure phase of the nanocrystals; for example, from a Laplace perspective the size dependence of surface tension for a nanocrystal can be formulated as22
γ(r) ) c1 + c2/r
(1)
where c1 represents the intrinsic energy associated with a flat index surface of a given composition, c2 equals the energy required to bend that material about edges, corners, etc., and r is the nanoparticle radius. For maximum achievable pressures of 180 kbar, changes in the optical density of the absorption spectrum of these doped Si nanocrystallites are on the order of 4% for the 3.3 nm sample and 6% for the 5.9 nm material and thus do not reflect a phase transition. While remaining elevated relative to the bulk value of 120 kbar, it is clear that the presence of erbium dopants in the nano-Si lattice does not significantly lower the observed phase transition pressure expected for nanophase Si (>200 kbar). As a control, we have also evaluated pressure-induced changes in optical density in this pressure regime for undoped Si nanocrystals of either 9 or 12 nm mean particle diameter. These Si nanoparticles yield comparable behavior, with negligible changes (1-2%) in integrated OD up to a maximum pressure of 180 kbar. The fact that Si nanocrystallites deliberately doped with erbium still exhibit an elevated phase transition is significant. It appears that the Er dopant does not introduce the type of structural defects that would lower the energy barrier to such a transformation, i.e., an absence of seeds for initiating nucleation of phase transitions. Furthermore, the barrier to structural transformation remains high because of the shape change associated with the formation of high-index, high-energy
surfaces;12 whatever the energy associated with the Er3+ centers, it does not lower this barrier in a substantial way. In summary, the optical absorption of Er3-doped Si nanocrystals has been studied as a function of high pressure in a diamond anvil cell. With increasing pressure, the onset of absorption in the UV-visible spectrum of Er3+-doped Si nanocrystals is shown to reversibly shift to lower energy at a rate comparable to that of bulk crystalline Si, inferring that the band-edge states reflect the bulklike character of the crystalline Si core rather than surface states. Structurally, Er3+-doped Si nanocrystals show (in a manner similar to undoped Si nanoparticles) no evidence for a phase transition up to a pressure of 180 kbar. This infers that the relative structural quality of these Er-doped nanocrystals is comparable to that of homogeneous oxide-capped Si nanocrystals grown by aerosol methods. Acknowledgment. Financial support by the National Science Foundation (DMR 98-19178) and the Robert A. Welch Foundation is gratefully acknowledged. References and Notes (1) Paul, W.; Warschauer, D. M. J. Phys. Chem. Solids 1958, 5, 102. (2) Slyhouse, T. E.; Drickamer, H. G. J. Phys. Chem. Solids 1958,7, 210. (3) Minomura, S.; Drickamer, H. G. J. Phys. Chem. Solids 1963, 23, 451. (4) Jamieson, J. C. Science 1963, 139, 762. (5) Wentorf, R. H.; Kasper, J. S. Science 1963, 139, 338. (6) Bundy, F. P. J. Chem. Phys. 1964, 41, 3809. (7) Welber, B.; Kim, C. K.; Cardona, M.; Rodriguez, S. Solid State Commun. 1975, 17, 1021. (8) Olijnyk, H.; Sikka, S. K.; Holzapfel, W. B. Phys. Lett. 1984, 103A, 137. (9) McMahon, M. I.; Nelmes, R. J. Phys. ReV. B 1993, 47, 8337. (10) McMahon, M. I.; Nelmes, R.; Wright, N. G.; Allan, D. R. Phys. ReV. B 1994, 50, 739. (11) Crain, J.; Ackland, G.; Maclean, J.; Piltz, R.; Hatton, P.; Pawley, G. Phys. ReV. B 1994, 50, 13043. (12) Tolbert, S.; Herhold, A.; Brus, L.; Alivisatos, A. Phys. ReV. Lett. 1996, 76, 4384. (13) Hanfland, M.; Schwartz, U.; Syassen, K.; Takemura, K. Phys. ReV. Lett. 1999, 82, 1197. (14) Nelmes, R. J.; McMahon, M. I. In Semiconductors and Semimetals; Suski, T., Paul, W., Ed.; Academic Press: New York, 1998; Vol. 1, pp 177-181. (15) Yeh, C. Y.; Zhang, S. B.; Zunger, A. Appl. Phys. Lett. 1993, 63, 3455. (16) Wang, L.; Zunger, A. J. Phys. Chem. 1994, 98, 2158. (17) Yeh, C. Y.; Zhang, S. B.; Zunger, A. Appl. Phys. Lett. 1994, 64, 3545. (18) St. John, J.; Coffer, J.; Chen, Y.; Pinizzotto, R. F. J. Am. Chem. Soc. 1999, 121, 1888. (19) Barnett, J. D.; Block, S.; Piermarini, G. J. ReV. Sci. Instrum. 1973, 44, 1. (20) Littau, K.; Szajowski, P.; Muller, A.; Kortan, A.; Brus, L. J. Phys. Chem. 1993, 97, 1224. (21) Pankove, J. Optical Processes in Semiconductors; Prentice-Hall; Englewood Cliffs, NJ, 1971; pp 37-42. (22) Tolbert, S. H.; Alivisatos, A. P. Annu. ReV. Phys. Chem. 1995, 46, 595.
