Interaction of One-Dimensional Photonic Crystals and Metal Nanoparticle Arrays and Its Application for Surface-Enhanced Raman Spectroscopy

We introduce a new concept to localize and strongly enhance electromagnetic fields by covering one-dimensional photonic crystals with ordered metal nanoparticles arrays. When designed properly, the combined photonic–plasmonic composite shows a significant interaction of the plasmonic resonance and the photonic band gap. For this purpose we fabricated one-dimensional photonic crystals based on porous silicon by electrochemical etching of silicon in hydrofluoric acid and deposited a silver nanoparticle array on top by nanosphere lithography. The composite structure was designed in such a way that the plasmonic resonance coincides with the photonic band gap, leading to highly confined electromagnetic fields at the interface between both structures. The samples were characterized using spectroscopic ellipsometry and reflectance measurements and were modeled using effective medium theories and finite-element methods. Surface-enhanced Raman spectroscopy measurements of this unique photonic–plasmonic hybrid system show extraordinary enhancement factors that can be explained only by an interaction mechanism. The optical properties of the composite structure are very versatile, providing a promising platform for improved sensing applications and superior substrates for surface-enhanced Raman spectroscopy

Origin of the Broadband Photoluminescence of Pristine and Cu⁺/Ag⁺‑Doped Ultrasmall CdS and CdSe/CdS Quantum Dots

Ultrasmall (∼2 nm) copper­(I)- and silver­(I)-doped CdS and core/shell CdSe/CdS quantum dots (QDs) stabilized by Cd­(II) complexes with mercaptoacetate anions and ammonia were produced in aqueous solutions. The doped QDs emit broadband visible photoluminescence (PL) with a quantum yield reaching 10–12% for Cu⁺-doped QDs and 5–9% for Ag⁺-doped QDs. The broadband PL was described by a self-trapped exciton model as a sequence of phonon replicas of a zero-phonon emission line. The shape of the PL bands of CdS, Cu⁺-doped CdS QDs, and Ag⁺-doped CdS QDs was modeled by using the energies of optical phonons of CdS, CuS, and Ag₂S, respectively. The dependence of the average PL lifetime of both pristine and doped CdS and CdSe/CdS QDs on PL registration wavelength was interpreted in terms of the vibrational relaxation of the self-trapped exciton. The analysis of PL properties of different ultrasmall metal chalcogenide QDs showed that the broadband PL can be described by a general model which does not require the assumption of participation of charge-trapping lattice defects

Highly Localized Strain in a MoS₂/Au Heterostructure Revealed by Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been rapidly improved over the past decade and opened up opportunities to study phonon properties of materials at the nanometer scale. In this Letter, we report on TERS of an ultrathin MoS₂ flake on a nanostructured Au on silicon surface forming a two-dimensional (2D) crystal/plasmonic heterostructure. Au nanostructures (shaped in triangles) are prepared by nanosphere lithography, and then MoS₂ is mechanically exfoliated on top of them. The TERS spectra acquired under resonance conditions at 638 nm excitation wavelength evidence strain changes spatially localized to regions as small as 25 nm in TERS imaging. We observe the highest Raman intensity enhancement for MoS₂ on top of Au nanotriangles due to the strong electromagnetic confinement between the tip and a single triangle. Our results enable us to determine the local strain in MoS₂ induced during heterostructure formation. The maximum frequency shift of E_2g mode is determined to be (4.2 ± 0.8) cm^–1, corresponding to 1.4% of biaxial strain induced in the MoS₂ layer. We find that the regions of maximum local strain correspond to the regions of maximum topographic curvature as extracted from atomic force microscopy measurements. This tip-enhanced Raman spectroscopy study allows us to determine the built-in strain that arises when 2D materials interact with other nanostructures

A Fine Size Selection of Brightly Luminescent Water-Soluble Ag–In–S and Ag–In–S/ZnS Quantum Dots

A size-selected series of water-soluble luminescent Ag–In–S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by a precipitation technique. Up to 10–11 fractions of size-selected AIS (AIS/ZnS) QDs emitting in a broad color range from deep-red to bluish-green were isolated with the photoluminescence (PL) quantum yield reaching 47% for intermediate fractions. The size of the isolated AIS (AIS/ZnS) QDs varied from ∼2 to ∼3.5 nm at a roughly constant chemical composition of the particles throughout the fractions as shown by the X-ray photoelectron spectroscopy. The decrease of the mean AIS QD size in consecutive fractions was accompanied by an increase of the structural QD imperfection/disorder as deduced from a notable Urbach absorption “tail” below the fundamental absorption edge. The Urbach energy increased from 90–100 meV for the largest QDs up to 350 meV for the smallest QDs, indicating a broadening of the distribution of sub-bandgap states. Both the Urbach energy and the PL bandwidth of the size-selected AIS QDs increased with QD size reduction from 3–4 to ∼2 nm, and a distinct correlation was observed between these parameters. A study of size-selected AIS and AIS/ZnS QDs by UV photoelectron spectroscopy on Au and FTO substrates revealed their valence band level <i>E</i>_VB at ∼6.6 eV (on Au) and ∼7 eV (on FTO) pinned to the Fermi level of conductive substrates resulting in a masking of any possible size-dependence of the valence band edge position

Optical Absorption Imaging by Photothermal Expansion with 4 nm Resolution

For quite a long time, people thought of the diffraction limit of light as a fundamental unbreakable barrier that prevents seeing objects with sizes smaller than half the wavelength of light. Super-resolution optical methods and near-field optics enabled overcoming this limitation. Here we report an alternative approach based on tracking the photothermal expansion that a nano-object experiences upon visible light absorption, applied successfully in the characterization of samples with a spatial/lateral resolution down to 4 nm. Our device consists of an atomic force microscope coupled with a solid-state laser and a mechanical chopper synchronized with the natural oscillation mode of an in-house-made gold tip cantilever system. This configuration enhances the detection of nanostructures due to the intermittent light excitation and the consequent intermittent thermal expansion of the sample under investigation. The sensitivity and spatial resolution are further improved by the electric field enhancement due to the excitation of localized surface plasmons at the tip apex. Our concept is demonstrated by the analysis of a two-dimensional material (GaSe) on crystalline sp² carbon (graphite) and by an array of multiwalled carbon nanotubes lithographically designed in a SiO₂ matrix. The photothermal expansion originating from light absorption leads to an unprecedented spatial resolution for an optical absorption event imaged below 10 nm

Origin and Dynamics of Highly Efficient Broadband Photoluminescence of Aqueous Glutathione-Capped Size-Selected Ag–In–S Quantum Dots

The 2–3 nm size-selected glutathione-capped Ag–In–S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by precipitation/redissolution from an aqueous colloidal ensemble. The QDs reveal broadband photoluminescence (PL) with a quantum yield of up to 60% for the most populated fraction of the core/shell AIS/ZnS QDs. The PL band shape can be described by a self-trapped exciton model implying the PL band being a sequence of phonon replica of a zero-phonon line resulting from strong electron–phonon interaction and a partial conversion of the electron excitation energy into lattice vibrations. It can be concluded that the position and shape of the PL bands of AIS QDs originate not from energy factors (depth and distribution of trap states) but rather from the dynamics of the electron–phonon interaction and the vibrational relaxation in the QDs. The rate of vibrational relaxation of the electron excitation energy in AIS QDs is found to be size-dependent, increasing almost twice from the largest to the smallest QDs

Determination of the Charge Transport Mechanisms in Ultrathin Copper Phthalocyanine Vertical Heterojunctions

Bulky organic semiconductors have been widely applied on a variety of devices including transistors, sensors, and organic light-emitting diodes. Recently, the capability of producing stable ultrathin organic semiconductor-based junctions has opened the possibility of a variety of novel device concepts, including high-speed organic transistors, organic spin valves, and biosensors. In such context, the investigation of the charge transport mechanisms across ultrathin organic semiconductors is the key for the engineering of emerging organic-based technologies. Here, the charge transport mechanisms across heterojunctions based on physisorbed ultrathin copper phthalocyanine on gold are precisely determined and controlled over a wide range of temperatures and electric fields. We observe that the macroscopic electrical characteristics of Au/CuPc/Au heterojunctions are similar to what has been reported for chemisorbed molecular junctions. For instance, the transition from thermally activated transport to tunneling is verified regardless of the nature of the molecule-contact bonding. The Au/CuPc/Au heterojunction transport is dominated by charge localization sites at high temperatures and, upon cooling, a continuous transition from direct tunneling, via resonant tunneling, to field emission takes place by increasing the voltage bias. Such a continuous transition has not been reported for a hybrid metal/organic heterojunction yet. We have also determined the dielectric constant of the CuPc molecular layer via transport measurements, which allowed us to infer the possible molecule arrangements between the electrodes

Stable Dispersion of Iodide-Capped PbSe Quantum Dots for High-Performance Low-Temperature Processed Electronics and Optoelectronics

Here, we present a ligand exchange of long insulating molecules with short, robust, and environmentally friendly iodide ions via a mild flocculation of PbSe nanocrystals (NCs). This ligand exchange leads to the formation of stable colloidal solutions in various polar solvents and in a broad concentration range via electrostatic repulsion. The iodide capping ligands preserve the electronic structure and maintain the optical properties of the PbSe NCs, both in solution and in the form of solid films. The spin-coated PbSe NC solids exhibit good transport characteristics with electron mobilities in the linear and saturation regimes reaching (2.1 ± 0.3) cm²/(V·s) and (2.9 ± 0.4) cm²/(V·s), respectively. This opens up opportunities for the low-cost and low-temperature fabrication of NC thin films being attractive for applications in the fields of electronics and optoelectronics

Doping-Induced Polaron Formation and Solid-State Polymerization in Benzoporphyrin–Oligothiophene Conjugated Systems

Benzoporphyrins and their derivatives are of high interest in organic semiconductor technology due to their peculiar physical properties valuable for optoelectronic applications. Following our previous work successfully developing <i>meso</i>-thienyl- or <i>meso</i>-bithiophenyl-substituted zinc benzoporphyrins as efficient ternary components for bulk heterojunction solar cells, we describe herein detailed spectroscopic studies on doping of solid films of these benzoporphyrins under iodine atmosphere. Solid-state doping and oxidative polymerization are investigated by Raman and Fourier transform infrared spectroscopy. Structural and vibrational changes upon doping are explored with supporting data from density functional theory calculations. Furthermore, the optical and spectroscopic characteristics of the films of these materials are also monitored during the doping, and the polaron formation as evidenced by in situ attenuated total reflection Fourier transform infrared and UV–vis spectroscopy is observed. These results suggest that the target zinc benzoporphyrins, both in monomeric and in polymeric forms, should be good candidates in several other optoelectronic applications

Confirming the Dual Role of Etchants during the Enrichment of Semiconducting Single Wall Carbon Nanotubes by Chemical Vapor Deposition

The search for ways to synthesize single wall carbon nanotubes (SWCNT) of a given electronic type in a controlled manner persists despite great challenges because the potential rewards are huge, in particular as a material beyond silicon. In this work we take a systematic look at three primary aspects of semiconducting enriched SWCNT grown by chemical vapor deposition. The role of catalyst choice, substrate, and feedstock mixture are investigated. In terms of semiconducting yield enhancement, little influence is found from either the binary catalyst or substrate choice. However, a very clear enrichment is found as one adds nominal amounts of methanol to an ethanol feedstock. Yields of up to 97% semiconducting SWCNT are obtained. These changes are attributed to two known etchant processes. In the first, metal SWCNT are preferentially etched. In the second, we reveal etchants also preferentially etch small diameter tubes because they are more reactive. The etchants are confirmed to have a dual role, to preferentially etch metallic tubes and narrow diameter tubes (both metallic and semiconducting) which results in a narrowing of the SWCNT diameter distribution