Martin Ledinský

Raman Spectroscopy of Organic–Inorganic Halide Perovskites

Micro-Raman spectroscopy provides laterally resolved microstructural information for a broad range of materials. In this Letter, we apply this technique to tri-iodide (CH3NH3PbI3), tribromide (CH3NH3PbBr3), and mixed iodide-bromide (CH3NH3PbI3-xBrx) organic-inorganic halide perovskite thin films and discuss necessary conditions to obtain reliable data. We explain how to measure Raman spectra of pristine CH3NH3PbI3 layers and discuss the distinct Raman bands that develop during moisture-induced degradation. We also prove unambiguously that the final degradation products contain pure PbI2. Moreover, we describe CH3NH3PbI3-xBrx Raman spectra and discuss how the perovskite crystallographic symmetries affect the Raman band intensities and spectral shapes. On the basis of the dependence of the Raman shift on the iodide-to-bromide ratio, we show that Raman spectroscopy is a fast and nondestructive method for the evaluation of the relative iodide-to-bromide ratio.

Effective Extraction of Photoluminescence from a Diamond Layer with a Photonic Crystal

Diamond-based materials possess many unique properties, one of them being a broad-band visible photoluminescence due to a variety of color centers. However, a high material refractive index makes the extraction of photoluminescence (PL) from a diamond layer inefficient. In this paper, we show that by periodical nanopatterning of the films surface into a form of two-dimensional photonic crystal, the extraction of PL can be strongly enhanced within the whole visible spectrum compared to the extraction of PL in a pristine or randomly nanopatterned film. On the basis of theoretical calculations, enhancement is shown to be due to the photonic crystal effect, including efficient coupling of an excitation laser into the diamond.

The structure and growth mechanism of Si nanoneedles prepared by plasma-enhanced chemical vapor deposition

Silicon nanowires and nanoneedles show promise for many device applications in nanoelectronics and nanophotonics, but the remaining challenge is to grow them at low temperatures on low-cost materials. Here we present plasma-enhanced chemical vapor deposition of crystalline/amorphous Si nanoneedles on glass at temperatures as low as 250 °C. High resolution electron microscopy and micro-Raman spectroscopy have been used to study the crystal structure and the growth mechanism of individual Si nanoneedles. The H(2) dilution of the SiH(4) plasma working gas has caused the formation of extremely sharp nanoneedle tips that in some cases do not contain a catalytic particle at the end.

Characterization of the mechanical properties of qPlus sensors

Summary In this paper we present a comparison of three different methods that can be used for estimating the stiffness of qPlus sensors. The first method is based on continuum theory of elasticity. The second (Cleveland’s method) uses the change in the eigenfrequency that is induced by the loading of small masses. Finally, the stiffness is obtained by analysis of the thermal noise spectrum. We show that all three methods give very similar results. Surprisingly, neither the gold wire nor the gluing give rise to significant changes of the stiffness in the case of our home-built sensors. Furthermore we describe a fast and cost-effective way to perform Cleveland’s method. This method is based on gluing small pieces of a tungsten wire; the mass is obtained from the volume of the wire, which is measured by optical microscopy. To facilitate detection of oscillation eigenfrequencies under ambient conditions, we designed and built a device for testing qPlus sensors.

Synthesis, structure, and opto-electronic properties of organic-based nanoscale heterojunctions

Enormous research effort has been put into optimizing organic-based opto-electronic systems for efficient generation of free charge carriers. This optimization is mainly due to typically high dissociation energy (0.1-1 eV) and short diffusion length (10 nm) of excitons in organic materials. Inherently, interplay of microscopic structural, chemical, and opto-electronic properties plays crucial role. We show that employing and combining advanced scanning probe techniques can provide us significant insight into the correlation of these properties. By adjusting parameters of contact- and tapping-mode atomic force microscopy (AFM), we perform morphologic and mechanical characterizations (nanoshaving) of organic layers, measure their electrical conductivity by current-sensing AFM, and deduce work functions and surface photovoltage (SPV) effects by Kelvin force microscopy using high spatial resolution. These data are further correlated with local material composition detected using micro-Raman spectroscopy and with other electronic transport data. We demonstrate benefits of this multi-dimensional characterizations on (i) bulk heterojunction of fully organic composite films, indicating differences in blend quality and component segregation leading to local shunts of photovoltaic cell, and (ii) thin-film heterojunction of polypyrrole (PPy) electropolymerized on hydrogen-terminated diamond, indicating covalent bonding and transfer of charge carriers from PPy to diamond.

Creating nanocrystals in amorphous silicon using a conductive tip

Field-enhanced metal-induced solid phase crystallization (FE-MISPC) of amorphous silicon is scaled down to nanoscale dimensions by using a sharp conductive tip in atomic force microscopy (AFM) as one of the electrodes. The room temperature process is driven by the electrical current of the order of 100 pA between the tip and the bottom nickel electrode. This results in energy transfer rates of 30-50 nJ s(-1). Amplitude of the current is limited by a MOSFET transistor to avoid electrical discharge from parasitic parallel capacitance. Limiting the current amplitude and control of the transferred energy (approximately 100 nJ) enables formation of silicon crystals with dimensions smaller than 100 nm in the amorphous film. Formation of the nanocrystals is localized by the AFM tip position. The presence of nanocrystals is detected by current-sensing AFM and independently corroborated by micro-Raman spectroscopy. The nanocrystal formation is discussed based on a model considering microscopic electrical contact, thermodynamics of crystallization and silicide formation.

Experimental quantification of useful and parasitic absorption of light in plasmon-enhanced thin silicon films for solar cells application

A combination of photocurrent and photothermal spectroscopic techniques is applied to experimentally quantify the useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (μc-Si:H) films incorporating optimized metal nanoparticle arrays, located at the rear surface, for improved light trapping via resonant plasmonic scattering. The photothermal technique accounts for the total absorptance and the photocurrent signal accounts only for the photons absorbed in the μc-Si:H layer (useful absorptance); therefore, the method allows for independent quantification of the useful and parasitic absorptance of the plasmonic (or any other) light trapping structure. We demonstrate that with a 0.9 μm thick absorber layer the optical losses related to the plasmonic light trapping in the whole structure are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. An average useful absorption of 43% and an average parasitic absorption of 19% over 400–1100 nm wavelength range is measured for μc-Si:H films deposited on optimized self-assembled Ag nanoparticles coupled with a flat mirror (plasmonic back reflector). For this sample, we demonstrate a significant broadband enhancement of the useful absorption resulting in the achievement of 91% of the maximum theoretical Lambertian limit of absorption.

Light trapping in thin-film solar cells measured by Raman spectroscopy

In this study, Raman spectroscopy is used as a tool to determine the light-trapping capability of textured ZnO front electrodes implemented in microcrystalline silicon (μc-Si:H) solar cells. Microcrystalline silicon films deposited on superstrates of various roughnesses are characterized by Raman micro-spectroscopy at excitation wavelengths of 442 nm, 514 nm, 633 nm, and 785 nm, respectively. The way to measure quantitatively and with a high level of reproducibility the Raman intensity is described in details. By varying the superstrate texture and with it the light trapping in the μc-Si:H absorber layer, we find significant differences in the absolute Raman intensity measured in the near infrared wavelength region (where light trapping is relevant). A good agreement between the absolute Raman intensity and the external quantum efficiency of the μc-Si:H solar cells is obtained, demonstrating the validity of the introduced method. Applications to thin-film solar cells, in general, and other optoelectronic devices are discussed.

Guided assembly of nanoparticles on electrostatically charged nanocrystalline diamond thin films

We apply atomic force microscope for local electrostatic charging of oxygen-terminated nanocrystalline diamond (NCD) thin films deposited on silicon, to induce electrostatically driven self-assembly of colloidal alumina nanoparticles into micro-patterns. Considering possible capacitive, sp2 phase and spatial uniformity factors to charging, we employ films with sub-100 nm thickness and about 60% relative sp2 phase content, probe the spatial material uniformity by Raman and electron microscopy, and repeat experiments at various positions. We demonstrate that electrostatic potential contrast on the NCD films varies between 0.1 and 1.2 V and that the contrast of more than ±1 V (as detected by Kelvin force microscopy) is able to induce self-assembly of the nanoparticles via coulombic and polarization forces. This opens prospects for applications of diamond and its unique set of properties in self-assembly of nano-devices and nano-systems.

Profilometry of thin films on rough substrates by Raman spectroscopy.

Thin, light-absorbing films attenuate the Raman signal of underlying substrates. In this article, we exploit this phenomenon to develop a contactless thickness profiling method for thin films deposited on rough substrates. We demonstrate this technique by probing profiles of thin amorphous silicon stripes deposited on rough crystalline silicon surfaces, which is a structure exploited in high-efficiency silicon heterojunction solar cells. Our spatially-resolved Raman measurements enable the thickness mapping of amorphous silicon over the whole active area of test solar cells with very high precision; the thickness detection limit is well below 1 nm and the spatial resolution is down to 500 nm, limited only by the optical resolution. We also discuss the wider applicability of this technique for the characterization of thin layers prepared on Raman/photoluminescence-active substrates, as well as its use for single-layer counting in multilayer 2D materials such as graphene, MoS2 and WS2.