arXiv subjects

Roman Krahne

Publications and source records attributed to Roman Krahne.

51 records · Page 3Linked to original sources

arXiv2020

Core/Shell CdSe/CdS Bone-Shaped Nanocrystals with a Thick and Anisotropic Shell as Optical Emitters

Colloidal core/shell nanocrystals are key materials for optoelectronics, enabling control over essential properties via precise engineering of the shape, thickness, and crystal lattice structure of their shell. Here, we apply the growth protocol for CdS branched nanocrystals on CdSe nanoplatelet seeds and obtain bone-shaped heterostructures with a highly anisotropic shell. Surprisingly, the nanoplatelets withstand the high growth temperature of 350 °C and we obtain structures with a CdSe nanoplatelet core that is overcoated by a shell of cubic CdS, on top of which tetrahedral CdS structures with hexagonal lattice are formed. These complex core/shell nanocrystals show a bandedge emission around 657 nm with a photoluminescence quantum yield of ca. 42 % in solution, which is also retained in thin films. Interestingly, the nanocrystals manifest simultaneous red and green emission, and the relatively long wavelength of the green emission indicates charge recombination at the cubic/hexagonal interface of the CdS shell. The nanocrystal films show amplified spontaneous emission, random lasing, and distributed feedback lasing when the material is deposited on suitable gratings. Our work stimulates the design and fabrication of more exotic core/shell heterostructures where charge carrier delocalization, dipole moment, and other optical and electrical properties can be engineered.

physics.chem-ph↗

arXiv2020

Composition, Size, and Surface Functionalization dependent Optical Properties of Lead Bromide Perovskite Nanocrystals

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on the surface passivation. We present a study on the temperature dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by the surface passivation, and a post-synthesis ligand exchange with a quaternary ammonium bromide (QAB) results in a more stable passivation over a larger temperature range. The PL intensity is highest from 50K-250K, which indicates that the ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), the surface passivation still needs improvement toward increased emission intensity in nanocrystal films.

physics.chem-ph↗

arXiv2020

Directional Anisotropy of the Vibrational Modes in 2D Layered Perovskites

The vibrational modes in organic/inorganic layered perovskites are of fundamental importance for their optoelectronic properties. The hierarchical architecture of the Ruddlesden-Popper phase of these materials allows for distinct directionality of the vibrational modes withrespect to the main axes of the pseudocubic lattice in the octahedral plane. Here, we study the directionality of the fundamental phonon modes in single exfoliated Ruddlesden-Popper perovskite flakes with polarized Raman spectroscopy at ultralow-frequencies. A wealth of Raman bands is distinguished in the range from 15-150 cm-1 (2-15 meV), whose features depend on the organic cation species, on temperature, and on the direction of the linear polarization of the incident light. By controlling the angle of the linear polarization of the excitation laser with respect to the in-plane axes of the octahedral layer, we gain detailed information on the symmetry of the vibrational modes. The choice of two different organic moieties, phenethylammonium (PEA) and butylammonium (BA) allows to discern the influence of the linker molecules, evidencing strong anisotropy of the vibrations for the (PEA)2PbBr4 samples. Temperature dependent Raman measurements reveal that the broad phonon bands observed at room temperature consist of a series of sharp modes, and that such mode splitting strongly differs for the different organic moieties and vibrational bands.

physics.chem-ph↗

arXiv2020

Nano- and microscale apertures in metal films fabricated by colloidal lithography with perovskite nanocrystals

We demonstrate patterning of metal surfaces based on lift-off of perovskite nanocrystals that enables the fabrication of nanometer-size features without the use of resist-based nanolithography. The perovskite nanocrystals act as templates for defining the shape of the apertures in metal layers, and we exploit the variety of sizes and shapes that can be controlled in the colloidal synthesis to demonstrate the fabrication of nanoholes, nanogaps and guides with size smaller than the wavelength of light in the visible spectrum. The process can be readily integrated with standard lithography and etching techniques for the creation of more complex structures.

physics.chem-ph↗

arXiv2020

Extending the colloidal transition metal dichalcogenide library to ReS2 nanosheets for application in gas sensing and electrocatalysis

Among the large family of transition metal dichalcogenides (TMDCs), recently ReS2 has stood out due to its nearly layer-independent optoelectronic and physicochemical properties. These are related to its 1T distorted octahedral structure, which leads to strong in-plane anisotropy and the presence of active sites at its surface, which makes ReS2 interesting for applications such as gas sensors and catalysts for H2 production. However, the current fabrication methods for ReS2 use chemical or physical vapor deposition (CVD or PVD) processes that are costly and involve complex and lengthy fabrication procedures, therefore limiting their large-scale production and exploitation. To address this issue, we developed a colloidal synthesis approach, which allows the production of ReS2 to be attained at temperatures below 360 Celsius degrees and with reaction times < 2 h, resulting in a more cost-efficient strategy than the CVD and PVD methods. By combining the solution-based synthesis with surface functionalization strategies, we demonstrate the feasibility of colloidal ReS2 nanosheet films for gas sensing of different toxic gases, moisture and other volatile compounds with highly competitive performance in comparison with devices built with CVD-grown ReS2 and MoS2. In addition, the integration of the ReS2 nanosheet films in assemblies, in which they are deposited on top of networks of carbon nanotubes, allowed us to fabricate electrodes for electrocatalysis for H2 production in both acid and alkaline conditions. Results from proof-of-principle devices show an electrocatalytic overpotential that is competitive with devices based on ReS2 produced by CVD, and even with MoS2, WS2 and MoSe2 electrocatalysts.

physics.app-ph↗

arXiv2019

Galvanic Replacement Reaction to prepare nanoporous Aluminum for UV plasmonics

Plasmonics applications have been extending into the ultraviolet region of the electromagnetic spectrum. Unfortunately the commonly used noble metals have intrinsic optical properties that limit their use above 350 nm. Aluminum is probably the most suitable material for UV plasmonics and in this work we show that nanoporous aluminum can be prepared starting from an alloy of Mg3Al2. The porous metal is obtained by means of a galvanic replacement reaction. Such a nanoporous metal can be exploited to achieve a plasmonic material for enhanced UV Raman spectroscopy and fluorescence. Thanks to the large surface to volume ratio this material represents a powerful platform for promoting interaction between plasmonic substrates and molecules in the UV.

physics.app-ph↗

arXiv2018

Single-mode lasing from colloidal water-soluble CdSe/CdS quantum dot-in-rods

Core-shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution-phase synthesis is based on surface-stabilizing ligands that makes them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties, poses a major challenge. This work demonstrates how core-shell CdSe/CdS quantum dot-in-rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light-emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot-in-rods. Films made from water-soluble dot-in-rods show amplified spontaneous emission (ASE) with a similar threshold (130 microJ/cm2) as the pristine material (115 microJ/cm2). To demonstrate feasibility for lasing applications, self-assembled micro lasers are fabricated via the coffee-ring effect that display single-mode operation and a very low threshold of around 10 microJ/cm2. The performance of these micro lasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot-in-rods.

physics.app-ph↗

arXiv2018

UV Light Detection from CdS Nanocrystal Sensitized Graphene Photodetectors at kHz Frequencies

We have fabricated UV-sensitive photodetectors based on colloidal CdS nanocrystals and graphene. The nanocrystals act as a sensitizer layer that improves light harvesting leading to high responsivity of the detector. Despite the slow relaxation of the photogenerated charges in the nanocrystal film, faster processes allowed to detect pulses up to a repetition rate of 2 kHz. We have performed time-resolved analysis of the processes occurring in our hybrid system, and discuss possible photo-induced charge transfer mechanisms.

cond-mat.mes-hall↗

arXiv2018

Multiband Plasmonic Sierpinski Carpet Fractal Antennas

Deterministic fractal antennas are employed to realize multimodal plasmonic devices. Such structures show strongly enhanced localized electromagnetic fields typically in the infrared range with a hierarchical spatial distribution. Realization of engineered fractal antennas operating in the optical regime would enable nanoplasmonic platforms for applications, such as energy harvesting, light sensing, and bio/chemical detection. Here, we introduce a novel plasmonic multiband metamaterial based on the Sierpinski carpet (SC) space-filling fractal, having a tunable and polarization-independent optical response, which exhibits multiple resonances from the visible to mid-infrared range. We investigate gold SCs fabricated by electron-beam lithography on CaF$_{2}$ and Si/SiO$_{2}$ substrates. Furthermore, we demonstrate that such resonances originate from diffraction-mediated localized surface plasmons, which can be tailored in deterministic fashion by tuning the shape, size, and position of the fractal elements. Moreover, our findings illustrate that SCs with high order of complexity present a strong and hierarchically distributed electromagnetic near-field of the plasmonic modes. Therefore, engineered plasmonic SCs provide an efficient strategy for the realization of compact active devices with a strong and broadband spectral response in the visible/mid-infrared range. We take advantage of such a technology by carrying out surface enhanced Raman spectroscopy (SERS) on Brilliant Cresyl Blue molecules deposited onto plasmonic SCs. We achieve a broadband SERS enhancement factor up to $10^{4}$, thereby providing a proof-of-concept application for chemical diagnostics.

physics.optics↗

arXiv2015

Electrical plasmon detection in graphene waveguides

We present a simple device architecture that allows all-electrical detection of plasmons in a graphene waveguide. The key principle of our electrical plasmon detection scheme is the non-linear nature of the hydrodynamic equations of motion that describe transport in graphene at room temperature and in a wide range of carrier densities. These non-linearities yield a dc voltage in response to the oscillating field of a propagating plasmon. For illustrative purposes, we calculate the dc voltage arising from the propagation of the lowest-energy modes in a fully analytical fashion. Our device architecture for all-electrical plasmon detection paves the way for the integration of graphene plasmonic waveguides in electronic circuits.

cond-mat.mes-hall↗

arXiv2013

CdSe-Au nanorod networks welded by gold domains - a promising structure for nano-optoelectronic components

CdSe-Au networks were synthesized by a colloidal chemistry technique. They entail CdSe nanorods with a diameter of ~10 nm and a length of ~40 nm, which are joined together by Au domains (~5 nm). Individual networks were positioned by AC dielectrophoresis between bow-tie electrodes with a gap of ~100 nm and their conductivity as well as the photoelectrical properties were investigated. Nanorod networks, with multiple Au domains on the nanorod surface, displayed stable conductivity that was not sensitive to blue laser light illumination. Such nanostructures were transformed by thermal annealing to networks with Au domains only at the nanorod tips. In this system the overall conductivity was reduced, but a clear photocurrent signal could be detected, manifesting semiconductor behavior.

cond-mat.mes-hall↗

arXiv2009

Self assembly of highly fluorescent semiconductor nanorods into large scale smectic liquid crystal structures by coffee stain evaporation dynamics

We deposit droplets of nanorods dispersed in solvents on substrate surfaces and let the solvent evaporate. We find that strong contact line pinning leads to dense nanorod deposition inside coffee stain fringes, where we observe large-scale lateral ordering of the nanorods with the long axis of the rods oriented parallel to the contact line. We observe birefringence of these coffee stain fringes by polarized microscopy and we find the direction of the extraordinary refractive index parallel to the long axis of the nanorods.

cond-mat.mtrl-sci↗

arXiv2002

Fabrication of Nano-Scale Gaps in Integrated Circuits

Nano-size objects like metal clusters present an ideal system for the study of quantum phenomena and for constructing practical quantum devices. Integrating these small objects in a macroscopic circuit is, however, a difficult task. So far the nanoparticles have been contacted and addressed by highly sophisticated techniques which are not suitable for large scale integration in macroscopic circuits. We present an optical lithography method that allows for the fabrication of a network of electrodes which are separated by gaps of controlled nanometer size. The main idea is to control the gap size with subnanometer precision using a structure grown by molecular beam epitaxy.

cond-mat.mes-hall↗

arXiv2001

From single dots to interacting arrays

We explore the structural changes in charge the density and the electron configuration of quantum dots caused by the presence of other dots in an array, and the interaction of neighboring dots. We discuss what recent measurements and calculation of the far-infrared absorption reveal about almost isolated quantum dots and investigate some aspects of the complex transition from isolated dots to dots with strongly overlapping electron density. We also address the the effects on the magnetization of such dot array.

cond-mat.mes-hall↗

arXiv2001

Far-Infrared Excitations below the Kohn Mode: Internal Motion in a Quantum Dot

We have investigated the far-infrared response of quantum dots in modulation doped GaAs heterostructures. We observe novel modes at frequencies below the center-of-mass Kohn mode. Comparison with Hartree-RPA calculations show that these modes arise from the flattened potential in our field-effect confined quantum dots. They reflect pronounced relative motion of the charge density with respect to the center-of-mass.

cond-mat.mes-hall↗