Optical properties of PbSe nanocrystal quantum dots under pressureKirill K. Zhuravlev, Jeffrey M. Pietryga, Robert K. Sander, and Richard D. Schaller Citation: Applied Physics Letters 90, 043110 (2007); doi: 10.1063/1.2431777 View online: http://dx.doi.org/10.1063/1.2431777 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Charge-transfer dynamics in multilayered PbS and PbSe quantum dot architectures Appl. Phys. Lett. 104, 051112 (2014); 10.1063/1.4863953 Electroabsorption spectra of PbSe nanocrystal quantum dots Appl. Phys. Lett. 98, 161911 (2011); 10.1063/1.3583450 Spectroscopy of the nonlinear refractive index of colloidal PbSe nanocrystals Appl. Phys. Lett. 89, 193106 (2006); 10.1063/1.2385658 Optical absorption in PbSe spherical quantum dots embedded in glass matrix J. Appl. Phys. 88, 750 (2000); 10.1063/1.373733 Direct formation of self-assembled quantum dots under tensile strain by heteroepitaxy of PbSe on PbTe (111) Appl. Phys. Lett. 73, 250 (1998); 10.1063/1.121770

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Optical properties of PbSe nanocrystal quantum dots under pressureKirill K. Zhuravlev, Jeffrey M. Pietryga, Robert K. Sander, and Richard D. Schallera�

Chemistry Division, Los Alamos National Laboratory, MS-J567, Los Alamos, New Mexico 87545

�Received 27 September 2006; accepted 12 December 2006; published online 24 January 2007�

The optical properties of PbSe nanocrystal quantum dots �NQDs� were studied as a function ofapplied hydrostatic pressure over the range from ambient to 5.4 GPa. PbSe NQDs exhibit an energygap that is dominated by quantum confinement. Despite such strong confinement, the authors findthat the energy gaps of 3, 5, and 7 nm diameter PbSe NQDs change monotonically with pressurewith a dependence that is almost entirely determined by the bulk deformation potential. The sizabledependence of the NQD energy gap with pressure invites applications in the areas of high speedpressure sensing and tunable IR lasers. © 2007 American Institute of Physics.�DOI: 10.1063/1.2431777�

The optical, electronic, and physical properties of semi-conductor nanocrystal quantum dots �NQDs� can differ sig-nificantly from those of the corresponding bulk material,which makes them attractive for a wide range of applica-tions. For instance, lead selenide �PbSe� NQDs exhibit asize-dependent absorption onset1 and bright, narrow-band in-frared �IR� photoluminescence �PL�,2 whereas bulk phasePbSe is essentially nonemissive. The optical properties ofNQDs as a function of applied pressure can be affected byboth bulk material properties as well as quantum confine-ment effects related to volume changes. Previous pressurestudies on CdSe NQDs reveal a system dominated by bulkproperties, as the optical band gap varies with pressure ac-cording to the bulk deformation potential.3,4 However, withan exciton Bohr radius of �6 nm, sub-10-nm CdSe NQDsexhibit only intermediate confinement effects �the hole is notquantum confined�.5 PbSe NQDs have small, nearly equalcarrier effective masses and a bulk exciton Bohr radius of46 nm such that both carriers experience very strong con-finement effects in sub-10- nm particles.6,7 Such strong con-finement �up to 70% or more of the total energy gap ofPbSe NQDs� can potentially impact the balance betweenbulk and nanoscale influences. Additionally, because PbSeNQDs can be made to emit with high efficiency��80% quantum yield7� and exhibit amplified stimulatedemission in the near-IR region,8 they may find utility as theactive medium for both high speed pressure sensors andpressure-tunable IR lasers. The pressure effects on the prop-erties of PbSe are, however, known for the bulk materialonly.9,10

In the present work, the optical properties of near-IRemitting PbSe NQD were studied as a function of pressure.Three NQD sizes, 3, 5, and 7 nm in diameter, were exam-ined, corresponding to ambient-pressure PL peaks at0.98 eV, 0.83 eV, and 0.71 eV, respectively. We measuredboth absorption and PL of PbSe NQDs in the range of pres-sures from ambient to more than 5 GPa. Both absorption andPL spectral features exhibit a redshift with increasing pres-sure. By relating the magnitude and direction of this shift tothat observed in bulk PbSe, we were able to draw conclu-sions about relative contributions of the bulk deformation

potential and quantum confinement effects to the observedenergy change.

PbSe NQDs synthesized according to Ref. 2 �size disper-sity of �5% –7%� were dissolved in deuterated chloroformor toluene and loaded into a Merrill-Bassett-type diamond-anvil cell �DAC�. The details of DAC loading and the mate-rial used for the gasket can be found elsewhere.11,12 A smallchip of ruby was placed in the DAC along with the materialunder investigation, and applied pressure was measured bythe usual method of observing the shift in the R1 line of rubyfluorescence to an accuracy of 0.05 GPa. The IR absorptionmeasurements were collected using a Nicolet Fourier-transform IR spectrometer using InGaAs and HgCdTe detec-tors. PL measurements were performed using an amplitude-modulated 808 nm laser. Signals were collected using a CaF2lens, dispersed with a 0.3 m spectrometer, and detected witha LN2-cooled InSb photodetector and lock-in amplifier. Allmeasurements were carried out at room temperature.

Examples of absorption and PL spectra of a single NQDsample at two different pressures are shown in Fig. 1, dem-onstrating the observed redshift of all spectral features withincreasing pressure. This shift is opposite in direction fromthat previously observed in CdSe NQDs, which has a posi-tive bulk deformation potential, but is consistent with thenegative deformation potential of bulk PbSe. In Fig. 2, theband gap energy, measured as the energy of the 1S excitonabsorption feature, is plotted versus pressure for 3, 5, and7 nm diameter PbSe NQDs. For all NQD sizes, a large shift

a�Electronic mail: rdsx@lanl.gov

FIG. 1. Infrared absorption �solid lines� and PL �dotted lines� spectra of5 nm diameter PbSe NQDs for two pressures: ambient �black� and 2.5 GPa�gray�. Spectra show a redshift of the lowest-energy 1S excitonic featureswith pressure.

APPLIED PHYSICS LETTERS 90, 043110 �2007�

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of absorption with pressure, 47, 54, and 56 meV/GPa, re-spectively, is observed. In Fig. 3, a similar trend can be ob-served for the dependence of the energy of the PL peak onpressure for 7 nm NQDs, exhibiting a redshift of similarmagnitude �44 meV/GPa�.

Recently, calculations of band structures and opticalproperties of lead chalcogenides have been performed in thedensity functional theory formalism,13 using the full-potential linear muffin-tin orbital method. These calculationsyield a pressure derivative of the band gap in bulk PbSe of−59.5 meV/GPa. Our experimental results are in relativelygood agreement with these findings. This consistency sug-gests that the contribution from volume reduction and theconcomitant increase in confinement energy due to the ap-plied pressure is minimal. As confinement energy is in-versely proportional to a quadratic function of the radius ofthe NQD, the applied pressure should shift the band gap tohigher energy. Our observations show the shift in the oppo-site direction, which indicates that confinement effects arenot the dominant factor in the pressure dependence, a resultthat is supported by straightforward calculations. We usedthe Murnaghan equation of state,14,15

D =D0

��PB0�/B0� + 1�1/3B0�, �1�

in order to determine the NQD diameter at a given pressure,using values for bulk modulus B0, its derivative B0�,

16,17 and azero-pressure diameter D0=7 nm. The value of the band gap

at a given NQD diameter D was then estimated via the fol-lowing expression:18

Eg�D� = Eg��� +1

0.0105D2 + 0.2655D + 0.0667. �2�

The calculated decrease in lattice parameter for 7 nm PbSeNQDs for a pressure of 4 GPa was �2%, which correspondsto a relative increase in confinement energy of approximately1.4%. Thus, despite the high confinement energy of ourNQDs �as can be inferred by comparison to the bulk bandgap of 0.26 eV�, the increase in confinement energy, in com-parison to the experimentally observed 30% decrease in op-tical band gap over the same pressure range, represents arelatively small contribution to the overall band gap pressuredependence.

However, according to Eq. �2�, the contribution to theslope dEg /dP from quantum confinement will become largerwith decreasing NQD size, although it does not become largeenough to reverse the dominant influence of the deformationpotential. In fact, we can see in Fig. 2 that there is a trend inthe slope dEg /dP as a function of NQD size, i.e., the slopebecomes smaller with decreasing NQD size. The total slope,which has contributions from bulk deformation potential andconfinement energy effects, becomes smaller with NQD size,since the terms are opposite in sign. Calculations usingEq. �2� yield the difference in the slope due to quantumconfinement for 3 and 7 nm NQDs to be approximately3.6 meV/GPa, as compared to the observed slope differenceof 9 meV/GPa, thus confirming both the sign and order ofmagnitude of the observed trend. Measurements of NQDsover a larger range of band gap energies could resolve theremaining discrepancy, as variation in material compressibil-ity with NQD size has been observed in similar materials.19

The pressure dependence of the band gap in this systemis very large compared to the shift in frequency of vibrationsin molecules as well as local vibrational modes in semi-conductors. The observed energy shift in PbSe NQDs isalso �60 times larger than that of ruby emission��0.9 meV/GPa� as well as �15% larger �opposite in sign�than the shift that is observed for CdSe NQDs.3 Additionally,PbSe NQDs exhibit bright, efficient emission and strong con-tinuum absorbance �see Fig. 1�, making PbSe NQDs an idealcandidate for applications such as pressure-tunable IR lasersor optical pressure sensing devices. NQD-based pressuresensing instruments should demonstrate additional advan-tages over conventional systems in applications requiring ex-tremely fast measurement, such as in measurement of shockwaves.20 The time for an impulse to transit the semiconduc-tor material is proportional to the size, so response times in aNQD-based system can be anticipated to be significantlyshorter than those employing bulk materials. Further, thefluorescence decay time of PbSe NQDs is orders of magni-tude shorter than that of conventional pressure sensing ma-terials such as ruby ��3.5 ms for ruby versus �1 �s forPbSe NQDs �Refs. 7 and 21��, substantially reducing re-sponse time. Finally, the shift of NQD absorbance featurescould also be used for pressure sensing, with the advantagethat, as absorption is an essentially instantaneous process,observations could be made on a time scale limited primarilyby instrumentation.

In conclusion, we performed IR absorption and PL mea-surements on PbSe NQDs under hydrostatic pressures up to5.4 GPa. It was found that all spectral features redshift with

FIG. 2. Peak position of the 1S absorption feature as a function of pressurefor 3 nm ���, 5 nm ���, and 7 nm ��� dots. The solid lines are linear fits tothe data; The dashed line is the calculated contribution from the change inconfinement for 7 nm dots.

FIG. 3. Photoluminescence peak position vs pressure for 7 nm diameterPbSe NQDs. The solid line is the linear fit to the data.

043110-2 Zhuravlev et al. Appl. Phys. Lett. 90, 043110 �2007�

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increasing pressure, in good agreement with recent calcula-tions. The contribution of the change in quantum confine-ment energy was small compared to the total change in bandgap energy; however, this contribution was found to increasein significance with decreasing NQD size, leading to an ob-servable size dependence of the band gap pressurederivative.

This work was supported by the U.S. Department of En-ergy under Contract No. W-7405-ENG-36 and the Intelli-gence Technology Innovation Center. One of the authors�J.M.P.� was supported by an Intelligence Community Re-search Fellowship. Another author �R.D.S.� was supportedby a Frederick Reines Fellowship.

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