Terahertz Electric Polarizability of Excitons in PbSe and CdSe Quantum Dots

Dakovski, Georgi L.; Lan, Song; Xia, Chen; Shan, Jie

doi:10.1021/jp069026o

Cited by 26 publications

(47 citation statements)

References 33 publications

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“…A more detailed analysis, taking into account the nonparabolicity of the electronic bands and the electron-hole interaction, yields / R 3:6 . Subsequent studies on PbSe quantum dots (Dakovski et al, 2007) revealed that similar conclusions can be drawn for PbSe QD systems. However, the THz response is no longer dominated by the holes due to the comparable band masses of electron and hole and corresponding similar level spacing for conduction and valence states in lead salts.…”

Section: Quantum Dotsmentioning

confidence: 58%

“…The origin of such a response (purely real dielectric response of photoexcited QDs) was subsequently revealed by Wang et al (2006) and Dakovski et al (2007). They argued that for QDs with strong confinement both electrons and holes occupy discrete energy levels separated by at least tens of meV (Efros and Rosen, 2000), i.e., energies significantly larger than the photon energies used to probe the levels (1 THz ¼ 4 meV); see Fig.…”

Section: Quantum Dotsmentioning

confidence: 98%

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Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

et al. 2011

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Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

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Section: Quantum Dotsmentioning

confidence: 58%

Section: Quantum Dotsmentioning

confidence: 98%

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

et al. 2011

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“…39 The macroscopic conductivity of mesoporous films can be described using effective medium theory, 39 and is characterized by a positive real conductivity (increasing with frequency) and a negative imaginary conductivity (decreasing with frequency). 39 Given the fact that the exciton polarizability in PbSe is exceptionally low 43 due to the high dielectric constant of PbSe ( ) 215), 45 the THz response is expected to be dominated by real conductivity resulting from free carriers in the TiO 2 -MPA-PbSe film. This expectation was confirmed by comparing the imaginary THz response for isolated PbSe QDs, and the real THz response for an unloaded TiO 2 film under similar excitation densities ( Figure S2 of the Supporting Information): the (imaginary) THz signal resulting from PbSe QDs is negligible compared to (real) response of free carriers in TiO 2 .…”

Section: Resultsmentioning

confidence: 99%

Spectroscopic Studies of Electron Injection in Quantum Dot Sensitized Mesoporous Oxide Films

et al. 2010

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Optimization of interfacial charge transfer in quantum dot (QD) sensitized mesoporous oxide films is crucial for the efficient design of QD sensitized solar cells (QDSSC). We employ TeraHertz time-domain spectroscopy (THz-TDS), transient absorption (TA) and time-resolved luminescence measurements, combined with transmission electron microscopy (TEM) and current-voltage measurements to study injection of electrons from PbSe QDs that are chemically linked to mesoporous oxide films. We illustrate that the interpretation of injection experiments is ambiguous for time-resolved optical measurements (TA and time-resolved luminescence) because nonradiative recombination processes at the oxide-QD interface have the same spectroscopic signature as electron injection. Complementary THz-TDS and current-voltage measurements demonstrate electron injection from the 1S e level of PbSe QDs into SnO 2 mesoporous films, but injection into TiO 2 films is not observed, although the time-resolved optical measurements could be interpreted as indicating injection. That injection takes place into SnO 2 but not into TiO 2 can be explained by the energy alignment of the 1S e level of the PbSe QD, which is energetically favorable and unfavorable respectively with respect to the oxide conduction band edges of SnO 2 and TiO 2 . THz-TDS experiments demonstrate that electron injection from 5.5 nm PbSe QDs into SnO 2 occurs on a time scale of 125 ( 40 ps. THz-TDS experiments further reveal a subensemble of photogenerated carriers in these QD-oxide systems that recombine within 10 ps after photoexcitation. These carriers are likely located in QD clusters formed on the oxide surface that are apparent from TEM images and where efficient nonradiative recombination processes occur. Control of QD cluster formation is therefore essential in the optimization of QDSSC devices because the time scale of carrier recombination in the QD clusters is shorter than the time scale of electron injection into the mesoporous oxide film.

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“…This is due to the real polarizability Re(α) = α′ of the excitons within the QDs, 3 which is proportional to Im[σ(ω)] = σ′(ω), according to 13 σ ω ω πε…”

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confidence: 99%

Loosening Quantum Confinement: Observation of Real Conductivity Caused by Hole Polarons in Semiconductor Nanocrystals Smaller than the Bohr Radius

et al. 2012

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ABSTRACT:We report on the gradual evolution of the conductivity of spherical CdTe nanocrystals of increasing size from the regime of strong quantum confinement with truly discrete energy levels to the regime of weak confinement with closely spaced hole states. We use the high-frequency (terahertz) real and imaginary conductivities of optically injected carriers in the nanocrystals to report on the degree of quantum confinement. For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the complex terahertz conductivity is purely imaginary. For nanocrystals with radii exceeding 5 nm, we observe the onset of real conductivity, which is attributed to the increasingly smaller separation between the hole states. Remarkably, this onset occurs for a nanocrystal radius significantly smaller than the bulk exciton Bohr radius a B ∼ 7 nm and cannot be explained by purely electronic transitions between hole states, as evidenced by tight-binding calculations. The real-valued conductivity observed in the larger nanocrystals can be explained by the emergence of mixed carrier-phonon, that is, polaron, states due to hole transitions that become resonant with, and couple strongly to, optical phonon modes for larger QDs. These polaron states possess larger oscillator strengths and broader absorption, and thereby give rise to enhanced real conductivity within the nanocrystals despite the confinement. KEYWORDS: Quantum dots, nanocrystals, polaron, quantum confinement, terahertz spectroscopy, intraband absorption, conductivity D ownsizing semiconductor structures into the nanometer scale is an important trend in electronic research and manufacturing, not only for the resulting increase in performance and compactness of electronic devices. Electronic quantum confinement occurring in materials possessing dimensions smaller than the charge carrier wave function provides a unique means for tailoring electronic properties. Among zero-dimensional semiconductor nanostructures, that is, quantum dots, colloidal nanocrystals have proven to be particularly useful in optoelectronic devices such as displays and solar cells, largely due to the tunability of the bandgap by size 1 in conjunction with simple solution processing and ample control over surface functionality.The degree of electronic confinement is commonly determined by the ratio between the nanocrystal radius R and the bulk exciton Bohr radius a B , and materials are classified accordingly into the strong (R/a B < 1), intermediate (R/a B ∼ 1), or weak (R/a B ≫ 1) quantum confinement regime. 2Colloidal semiconductor nanocrystals are typically manufactured to be in the strong confinement regime. Such nanoparticles are characterized by the occurrence of discrete, "atom-like" electronic states as a result of the spatial quantum confinement of electron and hole wave functions. The electronic properties of small nanoparticles are fundamentally different from their larger-sized (or bulk) counterparts, where electronic states form continuous bands. Electronic conduction in the classical sense ...

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Terahertz Electric Polarizability of Excitons in PbSe and CdSe Quantum Dots

Cited by 26 publications

References 33 publications

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Spectroscopic Studies of Electron Injection in Quantum Dot Sensitized Mesoporous Oxide Films

Loosening Quantum Confinement: Observation of Real Conductivity Caused by Hole Polarons in Semiconductor Nanocrystals Smaller than the Bohr Radius

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