Low Work Function Research Articles

Bulk ceramics of lanthanum hexaboride with enhanced spectral selectivity and photothermal efficiency for novel hybrid thermal-thermionic solar absorbers

Lanthanum hexaboride (LaB6) single-crystals are known from a long time for their excellent thermionic and field-assisted electron-emitting properties. Recently, multi-crystalline ceramics, which offer several technological advantages over single-crystals, have been studied as well, proving a low work function and Richardson constant, in addition to favorable optical properties for high-temperature solar radiation absorbers. However, to fully disclose to LaB6 ceramics the novel application of a double-function bulk thermionic emitter, enabled by the direct absorption of solar radiation, and, at the same time, thermodynamic solar absorber for Concentrating Solar Power (CSP), their optical properties should be further optimized. The ideal result, consisting in the selective increase of the solar absorptance and decrease of the thermal emittance, thus increasing both the spectral selectivity and the photothermal efficiency of the absorber, has been obtained in this work by realizing Laser-Induced Periodic Surface Structures (LIPSS) on the ceramic surface by means of femtosecond-laser treatments. LIPPS formation has been investigated as a function of the accumulated laser fluence.

Aluminum Halide-Based Electron-Selective Passivating Contacts for Crystalline Silicon Solar Cells.

Extensive research has focused on developing wide-bandgap metal compound-based passivating contacts as alternatives to conventional doped-silicon-layer-based passivating contacts to mitigate parasitic absorption losses in crystalline silicon (c-Si) solar cells. Herein, thermally-evaporated aluminum halides (AlX)-based electron-selective passivating contacts for c-Si solar cells are investigated. A low contact resistivity of 60.5 and 38.4 mΩ cm2 is obtained on the AlClx/n-type c-Si (n-Si) and AlFx/n-Si heterocontacts, respectively, thanks to the low work function of AlX. Power conversion efficiencies (PCEs) of 19.1% and 19.6% are achieved on proof-of-concept n-Si solar cells featuring a full-area AlClx/Al and AlFx/Al passivating contact, respectively. By further implementing an ultrathin SiO2 passivation interlayer and a pre-annealing treatment, the electron selectivity (especially the surface passivation) of AlX is significantly enhanced. Accordingly, a remarkable PCE of 21% is achieved on n-Si solar cells featuring a full-area SiO2/AlFx/Al rear contact. AlFx-based electron-selective passivating contacts exhibit good thermal stability up to ≈400 °C and better long-term environmental stability. This work demonstrates the potential of AlFx-based electron-selective passivating contact for solar cells.

Quantum capacitance, structural, optical and electronic properties of Zr2CT2 (T = F, P, Cl, Se, Br, O, S, OH) MXenes: A DFT study

Surface functionalization with termination functional groups plays a crucial role in the design of supercapacitors and electrode materials, whereas the understanding of the relevant mechanisms is still rather limited. Here, the quantum capacitance, work function and the related electronic and optical properties of eight functionalized MXenes Zr2CT2 monolayers (with functional group T = O, F, S, Cl, Br, OH, P and Se) are comprehensively investigated, by employing the first-principle calculations. All systems except Zr2CP2 are found to exhibit good stability. Modification with oxygen functional groups makes Zr2C change from metal to semiconductors. Surface functionalization modifies the work function of the MXenes by altering the Fermi level and the associated electron transfers. Particularly, the extremely low work function of Zr2C(OH)2 may enhance its power efficiency in optoelectronic fields. Introduction of functional groups may tune the density of electronic states near the Fermi level and effectively modulate the quantum capacitance and type of electrode materials. Zr2C suitable for symmetric supercapacitors and the modified systems applicable for potential anode or cathode materials are illustrated. The types of these electrode materials would not change in a wide voltage range. Further analyses on the optical properties and Bader charge are conducted for these materials.

Experimental and theoretical study on the facet‐dependent SERS activity of α‐Fe2O3 films

AbstractSemiconductors have attracted great attention for surface‐enhanced Raman scattering (SERS) applications due to their rich variety, adjustable band structure, good chemical stability, and biocompatibility. However, their intrinsic weak SERS activity limited the further development of semiconductor substrates. Here, the α‐Fe2O3 films with high {} and {} facet exposure ratios were fabricated through an electrodeposition method with the existence of NH4Cl and CH3COONa. The α‐Fe2O3{} and α‐Fe2O3{} films show excellent SERS properties with enhancement factor of 4.155×103 and 5.481×103, as well as the relatively low limit of detection down to 2 × 10−7 M for 4‐nitrobenzenethiol. Meanwhile, the lower relative standard deviation values (<10%) confirm the well uniformity of as‐prepared substrates. Ultraviolet photoelectron spectroscopy analysis reveals that the α‐Fe2O3{} possesses the lower work function, which could guarantee the efficient interfacial charge transfer between substrates and probe molecules. Moreover, first principles calculations further indicates that the charge transfer efficiency on {} facet can be effectively improved, and thus significantly enhance the SERS activity of α‐Fe2O3. The current study provides a strategy for the fabrication and application of semiconductor‐based SERS substrates.

Time dependence of SrVO3 thermionic electron emission properties

Single phase, polycrystalline perovskite oxide SrVO3, with its intrinsic low effective work function and facile synthesis process, is a promising thermionic electron emitter cathode candidate, in which previous works have shown evidence of an effective work function as low as 2.3 eV. In this work, we study the vacuum activation process of SrVO3 and find that it has promising emission stability over 15 days of continuous high temperature operation. We find that SrVO3 shows surface Sr and O segregation during its operation, which we hypothesize is needed to create a positive surface dipole, leading to a low effective work function. Emission repeatability from cyclic heating and cooling suggests the promising stability of the low effective work function surface, and additional observations of drift-free emission during 1 h of continuous emission testing at high temperature further demonstrate its excellent performance stability. This assessment of the emission stability over time and the interplay of evolving surface chemistry with emission behavior are necessary for understanding how best to prepare, process, and operate SrVO3 cathodes.

Influence of different terminations on the surface properties of MB2: first principles study

Metal diborides MB2 (M = Zr, V, Ta, Nb) are widely used in many optoelectronic devices. Investigation into their surface stability and morphology is the first and essential step in exploring their properties. In the present work, the periodic density functional theory method was performed to study the surface properties and optical properties of MB2 (M = Zr, V, Ta, Nb). The effects of different surface terminations are discussed, which indicates metal terminations have low work functions. The calculation of optical properties indicates that the four borides are suitable for spectral selective materials and optical storage materials. These findings provide valuable theoretical data for the design and application of MB2 in optoelectronic materials.

Beyond Conventional Carbon Activation: Creating Porosity without Etching Using Cesium Effect.

Facile synthesis of porous carbon with high yield and high specific surface area (SSA) from low-cost molecular precursors offers promising opportunities for their industrial applications. However, conventional activation methods using potassium and sodium hydroxides or carbonates suffer from low yields (<20%) and poor control over porosity and composition especially when high SSAs are targeted (>2000 m2 g-1) because nanopores are typically created by etching. Herein, a non-etching activation strategy is demonstrated using cesium salts of low-cost carboxylic acids as the sole precursor in producing porous carbons with yields of up to 25% and SSAs reaching 3008 m2 g-1. The pore size and oxygen content can be adjusted by tuning the synthesis temperature or changing the molecular precursor. Mechanistic investigation unravels the non-classical role of cesium as an activating agent. The cesium compounds that form in situ, including carbonates, oxides, and metallic cesium, have extremely low work function enabling electron injection into organic/carbonaceous framework, promoting condensation, and intercalation of cesium ions into graphitic stacks forming slit pores. The resulting porous carbons deliver a high capacity of 252 mAh g-1 (567 F g-1) and durability of 100000 cycles as cathodes of Zn-ion capacitors, showing their potential for electrochemical energy storage.

Photoelectron Spectroscopic Study of the Interfacial Electronic Structures of Metal‐Ion Containing Polyelectrolytes on ITO Substrates

AbstractResearch in the field of organic electronics has witnessed dramatic improvements in device performance over the past several decades through an ever‐improving understanding of electron and hole movement and the development of new interfacial materials. In this study, a type of interfacial material that relies on ionic charges comprising metal:poly(styrenesulfonate) (PSS) polyelectrolytes are synthesized and investigated as structural analogs of the ubiquitously used poly(3,4‐ethylenedioxythiophene:polystyrenesulfonate) (PEDOT:PSS) hole transport layer, in order to investigate correlations between metal cation ions and the cationic PEDOT component. The metal ions selected for this study include Li, Mg, V, Mn, Co, Ni, Cu, Zn, Pd, Ag, Cs, and Pb ions. To analyze the interfacial energy level alignment, electronic band structure, and band bending at the Indium tin oxide (ITO)/metal:PSS interface, X‐ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) are employed. Alkali (earth) and post‐transition metals show deep highest occupied molecular orbital (HOMO) levels and low work function (WF) due to Fermi level balance, implying poor hole transport. Remarkably, Cu:PSS displays a unique electronic structure, suggesting potential as a hole transport layer with increased WF and low hole injection barrier. Period 5 transition metals mirror PEDOT:PSS trends, and Ag:PSS holds the potential to form effective ohmic contacts.

Converting formaldehyde in methanol with MoO2 under irradiation: A pollution-free strategy for cleaning air

Direct photocatalytic reduction of toxic formaldehyde (HCHO) in value-added chemicals and fuels is promising because that not only abates the environmental pollution, but also solves the energy shortage. Herein, self-supported MoO2 and MoO3 nanoparticles growing on Mo meshes were comparatively applied to the photocatalytic conversion of HCHO. Under UV–visble lights, MoO2 reduces HCHO in methanol (CH3OH) while MoO3 oxidizes HCHO in carbon oxide and water. Their contrary photocatalytic capacities were revealed. Compared with MoO3, the lower work function of MoO2 enables an electron-rich interface, realizing a complete reduction of 30 ppm HCHO to CH3OH in 30 min. Theoretical calculations clarify that a large number of delocalized electrons on MoO2 attracts HCHO molecule and activates its CO bond, facilitating subsequent hydrogenation and reduction of HCHO to CH3OH. As for MoO3, the wider bandgap and higher potential of valence band govern the photocatalytic oxidation of HCHO.

Investigation of Laser-Induced Graphene (LIG) on a Flexible Substrate and Its Functionalization by Metal Doping for Gas-Sensing Applications.

Graphene materials synthesized using direct laser writing (laser-induced graphene; LIG) make favorable sensor materials because of their large surface area, ease of fabrication, and cost-effectiveness. In particular, LIG decorated with metal nanoparticles (NPs) has been used in various sensors, including chemical sensors and electronic and electrochemical biosensors. However, the effect of metal decoration on LIG sensors remains controversial; hypotheses based on computational simulations do not always match the experimental results, and even the experimental results reported by different researchers have not been consistent. In the present study, we explored the effects of metal decorations on LIG gas sensors, with NO2 and NH3 gases as the representative oxidizing and reducing agents, respectively. To eliminate the unwanted side effects arising from metal salt residues, metal NPs were directly deposited via vacuum evaporation. Although the gas sensitivities of the sensors deteriorate upon metal decoration irrespective of the metal work function, in the case of NO2 gas, they improve upon metal decoration in the case of NH3 exposure. A careful investigation of the chemical structure and morphology of the metal NPs in the LIG sensors shows that the spontaneous oxidation of metal NPs with a low work function changes the behavior of the LIG gas sensors and that the sensors' behaviors under NO2 and NH3 gases follow different principles.

Enhanced sunlight-driven photocatalysis owing to synergetic effect of gold nanoparticles-incorporated ZnO/rGO ternary heterostructures

The combination of gold nanoparticles (Au NPs) incorporated ZnO/rGO heterostructures is unique for sunlight-driven photocatalysis (SPC). Here, hydrothermally synthesized pristine ZnO nanostructures (NSs), ZnO/rGO binary nanocomposites (BNCs) and ZnO/rGO/Au ternary nanocomposites (TNCs) of bandgap (Eg) values 3.56 eV, 3.37 eV and 3.17 eV, respectively are reported. These NSs are analyzed optically, structurally and morphologically via UV–Vis absorption spectroscopy, X-ray diffraction and SEM techniques, respectively. The SPC of methylene blue (MB) organic dye was considered using all three samples. The slow charge transfer over grain boundaries of ZnO results in the prohibition of photogenerated electron-hole pairs, exhibiting low photodegradation efficiency (87%). However, in BNCs the creation of alternative pathway of the carriers due to low work function level of rGO as compared to ZnO enhances the charge separation with an increased photocatalytic efficiency (92%). In contrast, TNCs show rather fast photodegradation efficiency (96%) as the photoexcited electrons on Au NPs surface migrate to the CB of ZnO because of the SPR stimulation of Au. These electrons easily shift to the conductive rGO layers and react with oxygen species (O2) producing superoxide radicals (•O2–). Thus, TNCs sample has appeared to be auspicious candidate for the photodegradation of organic pollutants in untreated water.

Graphene/Hexagonal Boron Nitride Heterostructures for O2 Activation and CO Oxidation: Metal-Free Catalysts by Design.

Pristine graphene and h-BN monolayers are chemically inert to oxygen and thus exhibit very limited catalytic activity toward O2 activation. Herein, we show that graphene/h-BN heterostructures exhibit a surprising O2 activation capability. We theoretically designed ten graphene/h-BN heterostructures with three types of interfaces and investigated their catalytic activities toward O2 activation and CO-oxidation. In general, O2 can be molecularly chemisorbed and activated on electron-rich graphene/h-BN heterostructures. Electron-deficient graphene/h-BN heterostructures can lead to dissociative O2 adsorption with relatively low dissociation energy barriers (<0.4 eV). For CO-oxidation, the computed energy barrier can be as low as 0.67 eV. The high catalytic activities toward O2 stem from either electron-deficient heterostructures' accumulated electrons or electron richness and low work function for the electron-rich heterostructures. Although the catalytic activities of graphene/h-BN heterostructures depend strongly on the interface type, they are insensitive to the patterns of BN-substitutes, hence benefiting applicability of a wide range of heterostructures.

Fermi-level depinning in Mo/Si junctions by insertion of amorphous Si-rich Mo silicide film formed via gas-phase reactions of MoF6 with SiH4

We demonstrate the reduction of the electron Schottky barrier height (SBH) to 0.48 eV at Mo/n-type Si junctions through the insertion of a semimetal Si-rich Mo silicide (MoSi n , n = 7.9) film. Raman scattering measurements elucidated the persistence of the same amorphous structure within the MoSi n film even when subjected to temperatures as high as 900 °C. This excellent thermal stability yielded a notable result: preservation of the SBH modulation effect even following annealing at 700 °C. Moreover, we investigated the capacitance−voltage characteristics of MOS capacitors, revealing that MoSi n film has a remarkably low effective work function, measuring 4.1 eV when deposited onto SiO2. The deposition of MoSi n film was accomplished with an excellent coverage by using MoF6 and SiH4 gases. Thus, MoSi n film is a promising contact material in advanced CMOS transistors.

Post-annealing-free BaOxFy/LiF-based stack electron-selective contacts for high efficiency crystalline silicon solar cells featuring ultra-low contact resistivity

In silicon heterojunction and tunnel oxide passivating contact crystalline silicon (c-Si) solar cells based on successful passivating contacts technologies, an annealing activation step is indispensable to achieve a suitably low contact resistivity (ρc), which is a costly and non-ideal process. Here, combined with a calcium aluminium (Ca:Al) alloy electrode, a low work function barium oxide and lithium fluoride (BaOxFy/LiF) stack electron-selective contact (ESC) without an annealing activation step on c-Si solar cells clinches an ultra-low ρc of 1.4 mΩ·cm2 delivering a power conversion efficiency (PCE) of 20.5 %. The BaOxFy/LiF/Ca:Al stack structure is built by BaOx/LiF/Ca/Al hybrid structure via a facile continuous thermal evaporation process. As a proof of concept, the stacked BaOxFy/LiF/Ca:Al/Al contact is applied to the rear side of c-Si solar cells, achieving a PCE of 20.5 % with an open-circuit voltage of 625.7 mV, an impressive fill factor of 83.5 % and a short-circuit current density of 39.2 mA/cm2. With the aid of a transient photovoltage setup, the carrier recombination mechanism of the stacked BaOxFy/LiF/Ca:Al structure is explored. This work demonstrates an alternative idea for undoped ESCs, showing potential for next-generation cost-effective ultrathin-slice c-Si solar cells (∼80 μm) with high efficiency, and low-parasitic light absorption losses.

Design of a new process for the stabilization of FeS-Bi2S3 hybrid nanostructure and its application as a field emitter.

The relentless pursuit for technological advancement has fuelled intensive research into nanoarchitectures as fundamental components of various devices. One-dimensional (1-D) nanomaterials, including nanorods, nanowires, and nanotubes, have garnered significant attention due to their distinctive catalytic, optical, and electronic properties. Metal chalcogenides have emerged as promising candidates for diverse applications ranging from sensing devices to solar cells, particularly bismuth sulphide (Bi2S3). Bi2S3 exhibits unique properties owing to its low work function and anisotropic crystal structure. This work presents a novel approach to synthesize Bi2S3 nanorods and decorate them with FeS to form an FeS-Bi2S3 heterostructure via a one-step, template-free hydrothermal method. The synthesized nanomaterials are evaluated for their field emission characteristics, which are vital properties for numerous electronic applications. By comparing the field emission behaviour of the pristine Bi2S3 and FeS-Bi2S3 heterostructures, insights into the impact of hetero-structuring FeS for field emission performances are elucidated. This study presents insights into tailoring the heterostructure of different transition metals with Bi2S3 and studying their field emission behaviours.

Characterization of a LaB6 tip as a thermionically enhanced photoemitter

There is a widespread interest in time-resolved electron spectroscopies such as ultrafast electron diffraction, ultrafast electron microscopy, and ultrafast electron energy loss spectroscopy. These techniques require pulsed electron beams with both high current and brightness. LaB6 is commonly used as a thermionic emitter because of its low work function and high electron yield. However, its use as a pulsed photocathode has not been widely explored. Here, we present measurements of the electron yield from a LaB6 filament exposed to 392 nm UV ultrafast laser pulses under a wide range of filament temperatures. We find that sample heating strongly enhances photoelectron yield, an effect known as thermionically enhanced photoemission. However, it also creates potentially undesirable, continuous thermionic background. We conclude that the ideal optimal operating conditions strongly depend on the type of measurement and require defining and quantifying an appropriate figure of merit.

Electrical and thermal characteristics of LaB6 thermionic hollow cathodes operating in He, D2, Ar, and Xe at 4–200 Pa and 0.25–5 A cm−2

We report characteristics of lanthanum hexaboride (LaB6) cathodes for conditions relevant to a high-voltage, high-current switching device. LaB6 is selected because of its relatively low work function, stability in the presence of many common gas impurities, and a low evaporation rate (long life) at operating temperatures. We have investigated the effect of cathode geometry, gas type, pressure, and current on a LaB6 hollow cathode in a diode configuration with an opposing planar anode. The gas is stagnant at a relatively low pressure of 4–200 Pa (0.03–1.5 Torr) because of device high-voltage standoff requirements. In all cases, the plasma voltage decreases to a plateau value with increasing current where a practical device would operate. The plateau voltage aligns with the fill gas excitation or ionization voltage, depending on conditions. At high current, the cathode temperature exceeds that required for thermionic emission to supply the total current, a behavior that is understood by accounting for back-diffusing bulk electrons.

Characterisation of a Cs–Implanted Cu Photocathode

The generation of high-brightness electron beams is a crucial area of particle accelerator research and development. Photocathodes which offer high levels of quantum efficiency when illuminated at visible wavelengths are attractive as the drive laser technology is greatly simplified. The higher laser power levels available at longer wavelengths create headroom allowing use of manipulation techniques to optimise the longitudinal and transverse beam profiles, and so minimise electron beam emittance. Bi–alkali photocathodes which offer quantum efficiency ∼ 10 % under illumination at 532 nm are an example of this. Another solution is the use of modified photoemissive surfaces. Caesium has a low work function and readily photoemits when illuminated at green wavelengths (∼532nm). Caesium oxide has an even lower work function and emits at red wavelengths (∼635nm). We present data on our work to create a hybrid copper photocathode surface modified by implantation of caesium ions, measuring the surface roughness and probing its structure using MEIS. We measure the energy spread of photoemitted electrons, the QE as a function of illumination wavelength, and the practicality of this surface as a photocathode by assessing its lifetime on exposure to oxygen.

The tuned Schottky barrier of a CoP co-catalyst via the bridge of ohmic contact from molybdenum metal for enhanced photocatalytic hydrogen evolution

The Schottky barrier between CoP and g-C3N4 is tuned via nanometallic Mo with a low work function.

Improvement of Ti/Al/Ti Ohmic contacts on AlGaN/GaN heterostructures by insertion of a thin carbon interfacial layer

This Letter reports on the improvement of the morphological and electrical behavior in Ti/Al/Ti Ohmic contacts on AlGaN/GaN heterostructures by the insertion of a thin carbon interfacial layer. In particular, the presence of a carbon layer between the Ti/Al/Ti metal stack and the AlGaN surface leads to the lowering of the annealing temperature (down to 450 °C) required to obtain linear I–V curves and to the improvement of the contacts surface morphology. The temperature dependence of the specific contact resistance was explained by the thermionic field emission, with a reduction in the barrier height ΦB down to 0.62 eV in the annealed contacts with the interfacial carbon layer. The experimental evidence has been justified with the formation of a thin low work function TiC layer, which enhances the current conduction through the metal/AlGaN interface.