Vibrational spectroscopy, due to its inherent specificity in providing a spectral "fingerprint" of molecular composition, is an important tool to identify and quantify molecules in environmental, clinical, biotechnological, security, and manufacturing process applications, for example. Optical waveguide chips, offering mass production, robustness, sensitivity and the potential for widespread, low-cost deployment as sensors, are being exploited in two principal forms of vibrational spectroscopy, waveguide mid-infrared spectroscopy (WMIRS) and waveguide-enhanced Raman spectroscopy (WERS). For both, key strands of present research include reducing waveguide attenuation, increasing light/matter interaction strength, exploring methods to deal with complex samples such as using machine learning approaches and on-chip integration of photonic devices to enhance functionality and improve performance. The mid-infrared materials and devices for WMIRS are less well developed than the near-infrared materials and devices normally used for WERS, requiring focussed research into these aspects, including new sources and detectors, and into mitigating the water absorption which can dominate parts of the MIR spectrum. In the case of WERS, additional key strands of research include reducing and mitigating the background emission from waveguide materials, enhancing the Raman signal strength, and incorporating combined plasmonic nanometallic structures with WERS. Additionally, for WERS, the issue of sample complexity hindering molecular identification is being addressed by the inclusion of Raman reporters in assays. MWIRS has been successfully applied to gas sensing, detection of compounds in breath for cancer and infection diagnosis and to bacterial discrimination in aqueous samples, for example. WERS has recently been applied to the detection of antibiotics in plasma and, for example, to cardiac biomarkers. There is now a growing trend towards commercialisation of WERS in particular, which should lead to increased application to real samples and advances in the development of practical sensors. In this review, we focus on the trends in research in WMIRS and WERS in the past 2 years.

Integrating graphene and transition metal dichalcogenides (TMDs) into layered material heterostructures brings together the exciting properties that each constituent 2D material offers. However, scaling the growth of graphene-TMD and related heterostructures remains a major challenge. In this work, we demonstrate the use of electrodeposition with a single source precursor (SSP), WSeCl4, to grow few-layer WSe2 using graphene as an electrode. Through characterization via photoluminescence, X-ray photoelectron, and Raman spectroscopy, we show that the electrodeposited WSe2 is stoichiometric and exhibits semiconducting and light-emitting properties. TEM imaging was also performed to show the ordering of the stacked layers of WSe2 over graphene, demonstrating the polycrystalline structure of WSe2. This work paves the way toward utilizing electrodeposition to stack multiple TMDs, including MoS2, WS2, and WSe2 over graphene for electronic and optoelectronic applications.

There has been an enormous increase in the computational power readily available since the first numerical treatments of electrochemical problems in the early 1960s. This development has been accompanied by the development of powerful, widely available, commercial software modelling tools. Despite this, approximate analytical treatments remain extremely useful in the modelling of coupled diffusion/reaction problems in electrochemistry because of the insights they provide into the different possible behaviours of the system. In this paper we discuss the modelling of amperometric enzyme electrodes, taking as our exemplar redox hydrogel-based enzyme electrodes in which the enzyme is immobilized in a redox active polymer which wires the enzyme to the electrode. In this system the measured current is related to many different experimental variables including substrate concentration and diffusion coefficient, reaction rate constants, and film properties and thickness. The interplay of these factors is described and the role of Case diagrams in understanding coupled diffusion/reaction problems of this type is discussed.

The integration of graphene with other 2D materials has been extensively studied over the past decade to realize high-performance devices unattainable with single materials. Graphene-transition metal dichalcogenides (TMDCs) such as MoS2, WS2, MoSe2, and WSe2 vertical heterostructures have demonstrated promise in numerous electronic and optoelectronic applications due to the wide bandgap range and strong light-matter interaction in TMDCs, and the ability to form electrostatically tunable junctions with graphene. However, conventional methods for TMDCs growth, including chemical vapor deposition (CVD), electrodeposition, and atomic layer deposition (ALD), require high temperatures, which can degrade graphene's electrical and structural properties. Here, we investigate the impact of sulfur annealing on graphene, revealing significant etching and electrical degradation. Density functional theory (DFT) calculations identify the divacancy defect with two sulfur adatoms (DV-2S) and C-S-C bonds as the dominant defect, differing from the previously reported monovacancy with one sulfur adatom (MV-1S). This defect induces p-doping in graphene, consistent with experimental observations. To address these challenges, we introduce a protective strategy utilizing self-assembled monolayers (SAMs) during annealing, enabling the growth of high-quality WS2 on graphene via electrodeposition. Our findings provide a foundation for integrating TMDCs with graphene while preserving its properties, advancing high-performance electronic and optoelectronic applications.

Templated electrodeposition is an efficient technique for the bottom-up fabrication of nanostructures and can effectively control the size and shape of the electrodeposits. Here, mesoporous silica thin films with highly ordered mesopores and a regular three-dimensional mesostructure were synthesised as templates for electrodeposition. The mesoporous silica films have small mesopores (similar to 8 nm) and complex mesopore channels (Fmmm mesostructure with the [0 1 0] axis perpendicular to the substrate). Electrodeposition of bismuth, tellurium and bismuth-tellurium was investigated from electrolytes containing [(NBu4)-Bu-n][BiCl4], [(NBu4)-Bu-n](2)[TeCl6] and [(NBu4)-Bu-n]Cl dissolved in dry dichloromethane. Top-view SEM images showed Bi, Te and Bi doped-Te nanoparticles in the mesopores and cross-section SEM showed there were a few Te nanowires, in addition to the particle aggregations on the surface. This is a promising observation as it demonstrates the possibility of preparing sub-10 nm nanowires by templated electrodeposition even though the deposits are not uniformly electrodeposited in all the mesopores. EDX shows the deposited Bi-Te nanoparticles were tellurium-rich, XRD shows they were trigonal tellurium (ICSD 65692). A variety of parameters including the choice of pulsed electrodeposition conditions and [(NBu4)-Bu-n][BiCl4] concentration (2.25 mM and 3 mM) were investigated in order to control the composition of the deposit. All samples prepared by pulsed electrodeposition showed very low Bi:Te ratio (Bi/Te<0.02), whereas samples deposited for 5 min at -0.6 V achieved high Bi content (Bi/Te=0.49).

In this work, we present a method for direct, site-selective growth of tellurium nanowires by electrochemical deposition. The Te nanowires were grown laterally between two specially designed nanoband electrodes across a gap, and over a dielectric material, forming a lateral device structure directly. The resulting wires are crystalline and phase pure, as evidenced by Raman spectroscopy, EDS (energy dispersive X-ray spectroscopy), and ADF-STEM (annular dark field scanning transmission electron microscopy). The precise conditions for lateral growth of the nanowires were investigated and the fabrication of an electronic device from the as-deposited material, without the need for any transfer process or further contact fabrication, is demonstrated.

The process of electrochemically assisted surfactant assembly was followed in real time by grazing incidence small angle X-ray scattering with the aim to deconvolute the formation of mesoporous silica film and unwanted porous particles. The X-ray technique proved to be useful for the characterisation of this process, as it takes place at a very dynamic, solid/liquid interface. This paper shows the electrochemically driven onset and evolution of silica/surfactant structures. Additional control experiments indicate the formation of vertically aligned structures without the use of an electric field, although it seems to be beneficial for increased pore ordering.

A method for denoising Raman spectra is presented in this paper. The approach is based on the principle that the original signal can be restored by averaging pixels based on structure similarity. Similarity searching and averaging are not limited to the neighbouring pixels but extended throughout the entire signal range across different frames. This approach is distinguished from the conventional single-frame neighbour pixel-based filtering. The effectiveness and robustness of the proposed method are demonstrated through denoising simulated and experimental Raman data sets with fixed denoising parameters. Several denoised results and statistical indicators are presented for the simulated data. Recovery of the experimental Raman spectrum from our newly developed cost-effective waveguide-enhanced Raman spectroscopy system is also presented and compared to the spectrum from a conventional expensive Raman microscope for the same analyte.

The development of area-selective, scalable deposition methods for the anisotropic growth of transition metal dichalcogenide (TMDC) thin films with a planar morphology is essential for their practical applications in integrated electronic and optoelectronic devices. In this work, we report on the electrodeposition of layered WSe2 from a single source molecular precursor, containing both W and Se, for the first time. Using WSeCl4 in an acetonitrile (MeCN) electrolyte solution, we have employed cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) techniques to study the electrochemical behaviour of WSeCl4. A pulsed electrodeposition technique was then used to deposit WSe2 films, which possess a homogeneous composition across the whole electrode area. Characterization using scanning and transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirm the electrodeposited thin films to be WSe2. As a proof-of-concept for future growth directly into 3D device architectures, we present the 2D anisotropic growth of WSe2 thin films from the edge of a 100 nm thick TiN nanoband electrode across SiO2 on microfabricated 3D structures, allowing the direct measurement of electrical characteristics. Through this work, we also demonstrate electrodeposition as an area-selective growth technique suitable for obtaining highly anisotropic WSe2 thin films which are very promising for future electronic and optoelectronic applications. We report the electrodeposition of WSe2 thin films using a single source precursor (WSeCl4) and its 2D anisotropic growth from a microfabricated TiN nanoband electrode across an insulator, allowing direct electrical characterization.

Waveguide-enhanced Raman spectroscopy (WERS) exploits the electromagnetic enhancement that can be achieved at the surface of suitably designed waveguides to enhance the intensity of the Raman spectra of molecules close to the waveguide surface. This Primer describes practical aspects of WERS implementation including the choice of laser, choice of waveguide material, design and fabrication of the waveguides, coupling of light into and collection of light from the waveguide, and choice of spectrometer and filters. The methods for data collection and quantitative analysis of waveguide-enhanced Raman spectra are also described, together with the applications of WERS to problems in chemistry, materials science and bioscience. Issues of spectral reproducibility and key optimization factors are discussed together with a summary of technical limitations, current challenges and perspectives for future research. In many cases the material presented is supported by further, more detailed, discussion in the accompanying Supplementary Information. Waveguide-enhanced Raman spectroscopy (WERS) exploits the electromagnetic enhancement that can be achieved at the surface of suitably designed waveguides to enhance Raman intensity. In this Primer, Ettabib et al. describe methods for data collection and quantitative analysis of waveguide-enhanced Raman spectra.

Transition metal dichalcogenides (TMDCs) are an interesting group of 2D materials characterised with a layered structure analogous to graphene and they possess unique electronic and optical properties, especially when in the few- and mono-layer form. Developing scalable techniques for depositing TMDCs is a major challenge which needs to be overcome to fabricate functional devices with these materials. Electrodeposition is an industrially relevant technique that has some key advantages over other conventional deposition methods. It is a low cost and easily scalable technique and could be used for obtaining complex nanoscale features and for depositing over topologically demanding surfaces. Even though the electrodeposition of MoS2 has been achieved both in aqueous and non-aqueous electrolytes, not much progress has been made in the deposition of other TMDC materials. Tungsten based TMDCs such as WS2 and WSe2 are shown to be very promising materials in different applications, however electrodepositing them remains extremely challenging. One of the major obstacles here is developing electrochemically active precursors which are compatible with the electrolyte system and able to deliver both the tungsten and chalcogens to the electrolyte. Controlling and optimizing the deposition process to obtain few- and mono-layer TMDCs is another challenge. In addition, the choice of substrates for deposition is very important, especially for direct growth of ultra-thin TMDCs. Electrodeposition, being a bottom-up deposition method, would benefit from an atomically thin and smooth substrate such as graphene for depositing few- and mono-layer TMDCs. Here we present non-aqueous electroplating as a scalable alternative technique for tungsten-based TMDC deposition, with WS2 and WSe2 as examples. Tailored single source precursors were developed to use in non-aqueous electrolytes. WS2 was electrodeposited from dichloromethane (CH2Cl2) using the [NEt4]2[WS2Cl4] precursor. WSe2 was then electrodeposited from acetonitrile (CH3CN) electrolyte using [WSeCl4] as the precursor. Electrochemical quartz crystal microbalance (EQCM) studies were performed to optimize the deposition process and to probe the mechanism of precursor electrochemistry. Electrochemical deposition parameters were then carefully adjusted to obtain few- and mono-layer TMDCs. Patterned graphene electrodes were used as an atomically thin and smooth platform for the deposition of few- and mono-layer WS2. Few-layer TMDC films obtained on graphene were found to be much smoother than films deposited on other standard substrates such as titanium nitride (TiN) or Pt. These TMDC/graphene structures gave interesting 2D heterostructures which are technologically important for different applications. This work was funded by EPSRC grant references EP/V062689/1, EP/V062387/1 and EP/P025137/1

Electrodeposition of elemental Te from [NnBu4]2[TeCl6] in a weakly coordinating solvent, dichloromethane, has been investigated. The reduction of the Te(IV) complex to deposit Te shows a very large (> 1 V) overpotential on clean Pt macrodisc electrodes and microelectrodes that is significantly reduced once Te has been deposited on the electrode. Corresponding studies show that there is no significant overpotential for the deposition of Te from the Te(II) complex, [NEt4]2[TeI4], under the same conditions, suggesting that the barrier for electrodeposition from [TeCl6]2-arises because of the instability of the intermediate Te(III) species. Density functional theory (DFT) calculations with the M06-2X density functional and treatment of solvation using the polarizable continuum model confirm that the possible Te(III) complexes, [TeCl5]2-, [TeCl4]- and [TeCl3], are all unstable with respect to disproportionation, confirming that the Te(III) intermediate is a thermodynamic barrier to the electrodepo-sition of Te(0) from [TeCl6]2-. We propose that this thermodynamic barrier is overcome for the reaction at the Te electrode surface by a catalytic mechanism in which the transient Lewis acidic [TeCl5]- complex forms a Lewis acid/base adduct ([TeCl5]--Tesurface) through p-orbital overlap with the Te on the electrode surface. This mechanism is shown to be consistent with the results of DFT calculations for the speciation equilibria for the Te (IV) chloride complexes and with the results of experiments at different chloride concentration. Our results suggest that Lewis acid/base adduct formation at the electrode surface may also play a role in electrodeposition of other p-block elements.

Redox cycling (RC) is a powerful tool capable of amplifying faradaic currents in electroanalytical measurements, thus allowing an enhancement of sensitivity through fast multiple sequential oxidation and reduction reactions of a redox-active analyte. Present state-of-the-art RC devices are mostly based on planar electrode geometries either in 2D or 3D configurations, requiring cleanroom facilities and expensive microfabrication techniques. Here, the electrochemical elaboration and characterization of a 3D coaxial macroporous twin-electrode is reported, obtained by following a low-cost bottom-up approach. A nanoengineered highly organized porous material is the basis for the design of two threaded cylindrical porous gold microelectrodes with a gap in the micrometer range that can be fine-tuned. The potentials of the outer and inner electrodes are biased at values above and below the redox potential of the analyte so that a given molecule can participate several times in the electron exchange reaction by shuttling between both electrodes. The resulting signal amplification, combined with a straightforward synthesis strategy of the electrode architecture, allows envisioning numerous (bio)electroanalytical applications.

We report a waveguide-enhanced Raman spectroscopy (WERS) platform with alignment-tolerant under-chip grating input coupling. The demonstration is based on a 100-nm thick planar (slab) tantalum pentoxide (Ta2O5) waveguide and the use of benzyl alcohol (BnOH) and its deuterated form (d7- BnOH) as reference analytes. The use of grating couplers simplifies the WERS system by providing improved translational alignment tolerance, important for disposable chips, as well as contributing to improved Raman conversion efficiency. The use of non-volatile, non-toxic BnOH and d7-BnOH as chemical analytes results in easily observable shifts in the Raman vibration lines between the two forms, making them good candidates for calibrating Raman systems. The design and fabrication of the waveguide and grating couplers are described, and a discussion of further potential improvements in performance is presented.

Temperature is an important variable in electrochemistry, increasing the operating temperature has the capacity to provide significant increases in mass transport and electron transfer rates. In the case of electrodeposition, it can also allow the deposition of crystalline material which would otherwise be amorphous when grown at lower temperatures. In this work we exploit a high boiling point, weakly coordinating solvent, odichlorobenzene, to electrodeposit the p-block semiconductors antimony and antimony telluride at temperatures up to 140 & DEG;C. The effect of the temperature on the morphology and crystallinity of the deposits is investigated using scanning electron microscopy, X-ray diffraction, Raman spectroscopy and optical microscopy. An attempt is also made to rationalise the role of temperature in electrodeposition and its influence on the aforementioned properties.

The cathodic stripping of Te in CH2Cl2 with 0.1 M [NnBu4]Cl has been studied. In background electrolyte the cathodic stripping of Te behaves in the expected way. In contrast, when the stripping occurs in the [NnBu4]2[TeCl6] plating solution, unusual voltammetry is observed. Microelectrode and EQCM studies show that cathodic stripping of Te in the plating solution occurs with the passage of very little charge and leads to the formation of Te(0) in solution due to a rapid reaction between the reduced Te(-II) species and the Te(IV) salt. As a result, the limiting current for reduction of the Te(IV) is unchanged with 4 electron reduction leading to the formation of Te(0) on the electrode at potentials positive of the Te cathodic stripping potential and Te(0) in solution at potentials negative of the Te cathodic stripping potential. The same behaviour is found when a Te(II) precursor is used in place of the Te(IV) salt.

In this paper we report the use of Na-3[SbS4].9H(2)O as a single source precursor for the electrodeposition of Sb2S3 from aqueous electrolyte at pH 9.1. We present the electrochemistry of the [SbS4](3-)anion and the redox processes observed for the deposited Sb2S3 film. We show that an amorphous Sb2S3 film can be deposited by anodic electrodeposition onto glassy carbon and that the by-product that accompanies this deposition can be avoided by using a suitable pulse plating approach. Raman spectroscopy and grazing incidence X-ray diffraction were used to characterise the deposits and to show that good quality crystalline films of Sb2S3 are produced on annealing. The crystalline Sb2S3 films were screened for application in Resistive Random-Access Memory, and it was demonstrated that crystalline Sb2S3 films display typical bipolar resistive switching behaviour, and that the resistance ratio between the high resistance state and the low resistance state is approximately one order of magnitude at 1.5 V, which is sufficient for memory applications. A mechanism for the resistive switching is also proposed.

Metallic nanostructures have widespread applications in fields including materials science, electronics and catalysis. Mesoporous silica films synthesised by evaporation induced self-assembly and electrochemically assisted self-assembly with pores below 10 nm were used as hard templates for the electrodeposition of Au nanostructures. Electrodeposition conditions were optimised based on pore orientation and size. The growth of nanostructures was initiated at the electrode surface as confirmed by microscopy. The hard templates and Au electrodeposits were characterised electrochemically as well as with X-ray diffraction, small angle scattering and transmission electron microscopy. Finally, mesoporous silica hard templates were removed by hydrofluoric acid etching and stable Au nanoparticles on different electrode surfaces were achieved.

John Albery was one of the most creative, innovative and distinguished physical chemists of his time. He was particularly noted for his pioneering work in electrochemistry and electroanalytical chemistry, proton transfer kinetics and isotope effects, and enzyme kinetics. His DPhil studies on rotating disc electrodes led to seminal papers in the 1960s on ring-disc electrodes, which laid the basis for the extensive development of a powerful tool for electrochemical diagnostics. Those doctoral studies also involved work on the mechanisms of proton transfer reactions, a subject that remained very dear to his heart, and later resulted in the application of Marcus theory to provide understanding of physical organic reactions. In a separate strand on kinetic processes, he developed the theory of enzyme catalysis, which gave important insights into reaction kinetics of biological catalysts. He was a marvellous teacher and an inspiration to generations of his students, both undergraduate and post-graduate. His legacy reached far and wide, with a scientific family that included more than a dozen professorial ‘offspring’. His talents were not only restricted to science but extended to the theatre and entertainment, with writing for the satirical programme That was the week that was , the production of two musicals and of numerous departmental and college cabarets and irreverent annual revues. John Albery had an irrepressible joie de vivre and was the instigator of and participant in many occasions that were full of fun and laughter and live on in the memory. But it was not all fun and games; he had a keen sense of social justice. He led the assault to admit women to the previously men-only colleges of Oxford, and in the 1970s he mounted a vigorous and successful campaign to secure permission for the great Jewish electrochemist Benjamin Levich to leave the Soviet Union.

Memristors are emerging as promising candidates for practical application in reservoir computing systems that are capable of temporal information processing. Here, we experimentally implement a physical reservoir computing system using resistive memristors based on three-dimensional (3D)-structured mesoporous silica (mSiO2) thin films fabricated by a low cost, fast and vacuum-free sol-gel technique. The in situ learning capability and a classification accuracy of 100% on a standard machine learning dataset are experimentally demonstrated. The volatile (temporal) resistive switching in diffusive memristors arises from the formation and subsequent spontaneous rupture of conductive filaments via diffusion of Ag species within the 3D-structured nanopores of the mSiO2 thin film. Besides volatile switching, the devices also exhibit a bipolar non-volatile resistive switching behavior when the devices are operated at a higher compliance current level. The implementation of mSiO2 thin films opens the route to fabricate a simple and low cost dynamic memristor with a temporal information process functionality, which is essential for neuromorphic computing applications.