Pankaj Mandal

Terahertz Conductivity within Colloidal CsPbBr3 Perovskite Nanocrystals: Remarkably High Carrier Mobilities and Large Diffusion Lengths.

Colloidal CsPbBr3 perovskite nanocrystals (NCs) have emerged as an excellent light emitting material in last one year. Using time domain and time-resolved THz spectroscopy and density functional theory based calculations, we establish 3-fold free carrier recombination mechanism, namely, nonradiative Auger, bimolecular electron-hole recombination, and inefficient trap-assisted recombination in 11 nm sized colloidal CsPbBr3 NCs. Our results confirm a negligible influence of surface defects in trapping charge carriers, which in turn results into desirable intrinsic transport properties, from the perspective of device applications, such as remarkably high carrier mobility (∼4500 cm(2) V(-1) s(-1)), large diffusion length (>9.2 μm), and high luminescence quantum yield (80%). Despite being solution processed and possessing a large surface to volume ratio, this combination of high carrier mobility and diffusion length, along with nearly ideal photoluminescence quantum yield, is unique compared to any other colloidal quantum dot system.

Pulsed Nozzle Fourier Transform Microwave Spectrometer: Advances and Applications

Abstract The pulsed nozzle Fourier transform microwave (PNFTMW) spectrometer was developed by Balle and Flygare [A new method for observing the rotational spectra of weak molecular complexes: KrHCl. J. Chem. Phys. 1979, 71 (6), 2723–2724 and 1980, 72 (2), 922–932] in 1979. The design, fabrication, and operation of this spectrometer are complicated and it has largely remained a research laboratory tool till now, though a portable spectrometer for routine analytical applications has been developed at the National Institute for Standards and Technology [Suenram, R.D.; Grabow, J.‐U.; Zuban, A.; Leonov, I. A portable pulsed‐molecular‐beam Fourier‐transform microwave spectrometer designed for chemical analysis. Rev. Sci. Instrum. 1999, 70 (4), 2127–2135]. However, the potential for extracting fundamental information about any chemical species, such as, molecules, radicals, ions, or weakly bound complexes between any of them including atoms, has been quite significant. It is evident from the fact that more than 25 laboratories around the globe have built this spectrometer, some in the recent past. Contributions from all these laboratories have widened the horizon of PNFTMW spectrometers applications. This review summarizes the advances in design and the recent applications of this spectrometer. We also define an electrophore, as an atom/molecule that generates an electric dipole moment by forming a weakly bound complex with a species having zero electric dipole moment. The electrophore, thereby, enables structural determination using rotational spectroscopy, as in the case of Ar2–Ne, with Ne as the electrophore. Also, it can introduce a dipole moment about a principal axis where none existed before, such as in Ar–(H2O)2, enabling the observation of pure rotational transitions for several tunneling states.

Hydrogen bond radii for the hydrogen halides and van der Waals radius of hydrogen

In this article, the effective size of hydrogen in the hydrogen halides forming hydrogen bonded complexes is estimated. The scheme proposed by Bhadane and Gadre [J. Chem. Phys. 107, 5625 (1997)] for estimating the size of hydrogen in HF is extended to the other hydrogen halides (HCl and HBr) and HCN. It is noted that the radius of H atom in HF, HCl, HBr, and HCN are, respectively, 0.55±0.07, 0.74±0.08, 0.80±0.11, and 0.93±0.07 A. The radii found for HF, HCl, and HBr show a strong inverse correlation with the dipole moment of the HX. From this correlation the radius of H atom in HI is estimated to be 0.90±0.11 A. By extrapolating to zero dipole moment, the van der Waals radius of H atom is determined to be 1.0±0.1 A, reasonably close to the value proposed by Pauling, 1.2 A.

Intrinsic Rattler-Induced Low Thermal Conductivity in Zintl Type TlInTe2

Understanding the nature of chemical bonding and lattice dynamics together with their influence on phonon-transport is essential to explore and design crystalline solids with ultralow thermal conductivity for various applications including thermoelectrics. TlInTe2, with interlocked rigid and weakly bound substructures, exhibits lattice thermal conductivity as low as ca. 0.5 W/mK near room temperature, owing to rattling dynamics of weakly bound Tl cations. Large displacements of Tl cations along the c-axis, driven by electrostatic repulsion between localized electron clouds on Tl and Te ions, are akin to those of rattling guests in caged-systems. Heat capacity of TlInTe2 exhibits a broad peak at low-temperatures due to contribution from Tl-induced low-frequency Einstein modes as also evidenced from THz time domain spectroscopy. First-principles calculations reveal a strong coupling between large-amplitude coherent optic vibrations of Tl-rattlers along the c-axis, and acoustic phonons that likely causes the low lattice thermal conductivity in TlInTe2.

Terahertz Spectroscopic Probe of Hot Electron and Hole Transfer from Colloidal CsPbBr3 Perovskite Nanocrystals

Colloidal all inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have emerged to be an excellent material for applications in light emission, photovoltaics, and photocatalysis. Efficient interfacial transfer of photogenerated electrons and holes are essential for a good photovoltaic and photocatalytic material. Using time-resolved terahertz spectroscopy, we have measured the kinetics of photogenerated electron and hole transfer processes in CsPbBr3 NCs in the presence of benzoquinone and phenothiazine molecules as electron and hole acceptors, respectively. Efficient hot electron/hole transfer with a sub-300 fs time scale is the major channel of carrier transfer thus overcomes the problem related to Auger recombination. A secondary transfer of thermalized carriers also takes place with time scales of 20-50 ps for electrons and 137-166 ps for holes. This work suggests that suitable interfaces of CsPbX3 NCs with electron and hole transport layers would harvest hot carriers, increasing the photovoltaic and photocatalytic efficiencies.