APPLIED PHYSICS LETTERS 90, 043110 共2007兲
Optical properties of PbSe nanocrystal quantum dots under pressure Kirill K. Zhuravlev, Jeffrey M. Pietryga, Robert K. Sander, and Richard D. Schallera兲 Chemistry Division, Los Alamos National Laboratory, MS-J567, Los Alamos, New Mexico 87545
共Received 27 September 2006; accepted 12 December 2006; published online 24 January 2007兲 The optical properties of PbSe nanocrystal quantum dots 共NQDs兲 were studied as a function of applied hydrostatic pressure over the range from ambient to 5.4 GPa. PbSe NQDs exhibit an energy gap that is dominated by quantum confinement. Despite such strong confinement, the authors find that the energy gaps of 3, 5, and 7 nm diameter PbSe NQDs change monotonically with pressure with a dependence that is almost entirely determined by the bulk deformation potential. The sizable dependence of the NQD energy gap with pressure invites applications in the areas of high speed pressure sensing and tunable IR lasers. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2431777兴 The optical, electronic, and physical properties of semiconductor nanocrystal quantum dots 共NQDs兲 can differ significantly from those of the corresponding bulk material, which makes them attractive for a wide range of applications. For instance, lead selenide 共PbSe兲 NQDs exhibit a size-dependent absorption onset1 and bright, narrow-band infrared 共IR兲 photoluminescence 共PL兲,2 whereas bulk phase PbSe is essentially nonemissive. The optical properties of NQDs as a function of applied pressure can be affected by both bulk material properties as well as quantum confinement effects related to volume changes. Previous pressure studies on CdSe NQDs reveal a system dominated by bulk properties, as the optical band gap varies with pressure according to the bulk deformation potential.3,4 However, with an exciton Bohr radius of ⬃6 nm, sub-10-nm CdSe NQDs exhibit only intermediate confinement effects 共the hole is not quantum confined兲.5 PbSe NQDs have small, nearly equal carrier effective masses and a bulk exciton Bohr radius of 46 nm such that both carriers experience very strong confinement effects in sub-10- nm particles.6,7 Such strong confinement 共up to 70% or more of the total energy gap of PbSe NQDs兲 can potentially impact the balance between bulk and nanoscale influences. Additionally, because PbSe NQDs can be made to emit with high efficiency 共⬎80% quantum yield7兲 and exhibit amplified stimulated emission in the near-IR region,8 they may find utility as the active medium for both high speed pressure sensors and pressure-tunable IR lasers. The pressure effects on the properties of PbSe are, however, known for the bulk material only.9,10 In the present work, the optical properties of near-IR emitting PbSe NQD were studied as a function of pressure. Three NQD sizes, 3, 5, and 7 nm in diameter, were examined, corresponding to ambient-pressure PL peaks at 0.98 eV, 0.83 eV, and 0.71 eV, respectively. We measured both absorption and PL of PbSe NQDs in the range of pressures from ambient to more than 5 GPa. Both absorption and PL spectral features exhibit a redshift with increasing pressure. By relating the magnitude and direction of this shift to that observed in bulk PbSe, we were able to draw conclusions about relative contributions of the bulk deformation a兲
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potential and quantum confinement effects to the observed energy change. PbSe NQDs synthesized according to Ref. 2 共size dispersity of ⬃5 % – 7%兲 were dissolved in deuterated chloroform or toluene and loaded into a Merrill-Bassett-type diamondanvil cell 共DAC兲. The details of DAC loading and the material used for the gasket can be found elsewhere.11,12 A small chip of ruby was placed in the DAC along with the material under investigation, and applied pressure was measured by the usual method of observing the shift in the R1 line of ruby fluorescence to an accuracy of 0.05 GPa. The IR absorption measurements were collected using a Nicolet Fouriertransform IR spectrometer using InGaAs and HgCdTe detectors. PL measurements were performed using an amplitudemodulated 808 nm laser. Signals were collected using a CaF2 lens, dispersed with a 0.3 m spectrometer, and detected with a LN2-cooled InSb photodetector and lock-in amplifier. All measurements were carried out at room temperature. Examples of absorption and PL spectra of a single NQD sample at two different pressures are shown in Fig. 1, demonstrating the observed redshift of all spectral features with increasing pressure. This shift is opposite in direction from that previously observed in CdSe NQDs, which has a positive bulk deformation potential, but is consistent with the negative deformation potential of bulk PbSe. In Fig. 2, the band gap energy, measured as the energy of the 1S exciton absorption feature, is plotted versus pressure for 3, 5, and 7 nm diameter PbSe NQDs. For all NQD sizes, a large shift
FIG. 1. Infrared absorption 共solid lines兲 and PL 共dotted lines兲 spectra of 5 nm diameter PbSe NQDs for two pressures: ambient 共black兲 and 2.5 GPa 共gray兲. Spectra show a redshift of the lowest-energy 1S excitonic features with pressure.
0003-6951/2007/90共4兲/043110/3/$23.00 90, 043110-1 © 2007 American Institute of Physics Downloaded 25 Jan 2007 to 128.165.88.40. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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Appl. Phys. Lett. 90, 043110 共2007兲
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at a given NQD diameter D was then estimated via the following expression:18 Eg共D兲 = Eg共⬁兲 +
FIG. 2. Peak position of the 1S absorption feature as a function of pressure for 3 nm 共쎲兲, 5 nm 共䉱兲, and 7 nm 共䊏兲 dots. The solid lines are linear fits to the data; The dashed line is the calculated contribution from the change in confinement for 7 nm dots.
of absorption with pressure, 47, 54, and 56 meV/ GPa, respectively, is observed. In Fig. 3, a similar trend can be observed for the dependence of the energy of the PL peak on pressure for 7 nm NQDs, exhibiting a redshift of similar magnitude 共44 meV/ GPa兲. Recently, calculations of band structures and optical properties of lead chalcogenides have been performed in the density functional theory formalism,13 using the fullpotential linear muffin-tin orbital method. These calculations yield a pressure derivative of the band gap in bulk PbSe of −59.5 meV/ GPa. Our experimental results are in relatively good agreement with these findings. This consistency suggests that the contribution from volume reduction and the concomitant increase in confinement energy due to the applied pressure is minimal. As confinement energy is inversely proportional to a quadratic function of the radius of the NQD, the applied pressure should shift the band gap to higher energy. Our observations show the shift in the opposite direction, which indicates that confinement effects are not the dominant factor in the pressure dependence, a result that is supported by straightforward calculations. We used the Murnaghan equation of state,14,15
D=
D0 关共PB0⬘/B0兲 + 1兴1/3B0⬘
,
共1兲
in order to determine the NQD diameter at a given pressure, using values for bulk modulus B0, its derivative B0⬘,16,17 and a zero-pressure diameter D0 = 7 nm. The value of the band gap
1 . 0.0105D2 + 0.2655D + 0.0667
共2兲
The calculated decrease in lattice parameter for 7 nm PbSe NQDs for a pressure of 4 GPa was ⬃2%, which corresponds to a relative increase in confinement energy of approximately 1.4%. Thus, despite the high confinement energy of our NQDs 共as can be inferred by comparison to the bulk band gap of 0.26 eV兲, the increase in confinement energy, in comparison to the experimentally observed 30% decrease in optical band gap over the same pressure range, represents a relatively small contribution to the overall band gap pressure dependence. However, according to Eq. 共2兲, the contribution to the slope dEg / dP from quantum confinement will become larger with decreasing NQD size, although it does not become large enough to reverse the dominant influence of the deformation potential. In fact, we can see in Fig. 2 that there is a trend in the slope dEg / dP as a function of NQD size, i.e., the slope becomes smaller with decreasing NQD size. The total slope, which has contributions from bulk deformation potential and confinement energy effects, becomes smaller with NQD size, since the terms are opposite in sign. Calculations using Eq. 共2兲 yield the difference in the slope due to quantum confinement for 3 and 7 nm NQDs to be approximately 3.6 meV/ GPa, as compared to the observed slope difference of 9 meV/ GPa, thus confirming both the sign and order of magnitude of the observed trend. Measurements of NQDs over a larger range of band gap energies could resolve the remaining discrepancy, as variation in material compressibility with NQD size has been observed in similar materials.19 The pressure dependence of the band gap in this system is very large compared to the shift in frequency of vibrations in molecules as well as local vibrational modes in semiconductors. The observed energy shift in PbSe NQDs is also ⬃60 times larger than that of ruby emission 共⬃0.9 meV/ GPa兲 as well as ⬃15% larger 共opposite in sign兲 than the shift that is observed for CdSe NQDs.3 Additionally, PbSe NQDs exhibit bright, efficient emission and strong continuum absorbance 共see Fig. 1兲, making PbSe NQDs an ideal candidate for applications such as pressure-tunable IR lasers or optical pressure sensing devices. NQD-based pressure sensing instruments should demonstrate additional advantages over conventional systems in applications requiring extremely fast measurement, such as in measurement of shock waves.20 The time for an impulse to transit the semiconductor material is proportional to the size, so response times in a NQD-based system can be anticipated to be significantly shorter than those employing bulk materials. Further, the fluorescence decay time of PbSe NQDs is orders of magnitude shorter than that of conventional pressure sensing materials such as ruby 关艌3.5 ms for ruby versus ⬍1 s for PbSe NQDs 共Refs. 7 and 21兲兴, substantially reducing response time. Finally, the shift of NQD absorbance features could also be used for pressure sensing, with the advantage that, as absorption is an essentially instantaneous process, observations could be made on a time scale limited primarily by instrumentation. In conclusion, we performed IR absorption and PL measurements on PbSe NQDs under hydrostatic pressures up to 5.4 GPa. It was found that all spectral features redshift with
FIG. 3. Photoluminescence peak position vs pressure for 7 nm diameter PbSe NQDs. The solid line is the linear fit to the data. Downloaded 25 Jan 2007 to 128.165.88.40. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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increasing pressure, in good agreement with recent calculations. The contribution of the change in quantum confinement energy was small compared to the total change in band gap energy; however, this contribution was found to increase in significance with decreasing NQD size, leading to an observable size dependence of the band gap pressure derivative. This work was supported by the U.S. Department of Energy under Contract No. W-7405-ENG-36 and the Intelligence Technology Innovation Center. One of the authors 共J.M.P.兲 was supported by an Intelligence Community Research Fellowship. Another author 共R.D.S.兲 was supported by a Frederick Reines Fellowship. 1
Appl. Phys. Lett. 90, 043110 共2007兲
Zhuravlev et al.
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