Fermi Level Alignment Research Articles

Unprecedented Hole Concentration of NiSe2 Electrodes Leading to Hole Degeneration of Entire WSe2 Channels.

Semimetal electrodes for 2D semiconductors have been extensively studied; however, research on p-type semimetals has been limited due to the scarcity of materials that satisfy both high work functions and low resistances. In response, we investigated the behavior of NiSe2 as an electrode. Utilizing a novel co-evaporation method that suppresses oxidation and contamination, we synthesized NiSe2 demonstrating a high work function of 5.53 eV, a low resistance of 5.81 × 10-5 Ω cm, and a record hole concentration exceeding 1023 cm-3. We confirmed that when WSe2 comes into contact with NiSe2, the Fermi level of WSe2 falls below the work function of NiSe2, leading to complete hole degeneration of the channel. This unusual Fermi level alignment correlates with the high hole concentration in NiSe2 and the maximum energy level of its hole pockets. Our research provides new insights into how an electrode's hole concentration affects channel behavior.

Tip-induced local Fermi level alignment: A Stark shift in vacuum level in scanning tunneling microscope configurations

Electric fields in the junction of a scanning tunneling microscope (STM) are generally considered to have a negligible impact on the vacuum level (VL) of materials. We employed field emission resonance (FER) in the STM, combined with the triangular potential model, to measure the VL of the Ag(100) surface under varying electric currents. Unexpectedly, our results reveal that the VL exhibits a linear positive energy shift with increasing electric field strength. We suggest that this Stark shift in the VL arises from local Fermi level alignment induced by the STM tip. Additionally, we examined the VL of Ag islands grown on Cu(111) and Au(111) surfaces under different currents. Despite the Ag island having a lower work function than the Cu(111) and Au(111) surfaces, the energy shift in the VL with respect to the electric field on the Ag island is almost identical to that on the substrate under the same tip structure. This suggests that the Stark shift of the VL is insensitive to the work function. These findings are crucial for utilizing FER to measure local work function variations on surfaces, as the measured value is not influenced by the STM tip structure or the tunneling current, both of which can alter the electric field.

Organic thermoelectric device utilizing charge transfer interface as the charge generation by harvesting thermal energy

We propose an organic thermoelectric device having a new power generation mechanism that extracts small-scale thermal energy, i.e., a few tens of millielectronvolts, at room temperature without a temperature gradient. We demonstrate a new operating mechanism based on an organic thermoelectric power generation architecture that uses the charge separation capabilities of organic charge transfer (CT) interfaces composed of copper (II) phthalocyanine and copper (II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine as the donor and acceptor, respectively. With the optimized device architecture, values of open-circuit voltage VOC of 384 mV, short-circuit current density JSC of 1.1 μA/cm2, and maximum output Pmax of 94 nW/cm2 are obtained. The temperature characteristics of the thermoelectric properties yield activation energy values of approximately 20–60 meV, confirming the low-level thermal energy’s contribution to the power generation mechanism. Furthermore, from surface potential analysis using a Kelvin probe, we confirm that charges are generated at the CT interface, and the electrons and holes are diffused to the counter-electrodes with the aid of Fermi-level alignment between adjacent layers.

Halide double perovskites Bi2WO6/Cs2AgBiBr6 with efficient charge separation and wide visible light absorption for high activity photocatalysis

Wide visible light absorption and effective spatial charge separation are also pursued for efficient photocatalytic degradation. In this work, Bi2WO6/Cs2AgBiBr6 (BWO/CABB) composites were prepared by simple hydrothermal, ball milling and thermal evaporation methods, and studied structural and optical properties using various characterizations. The activity of the obtained BWO/CABB nanocomposites for visible light degradation of organic pollutants was significantly improved. The optimal 3BWO/CABB composite has an ethanol-phase degradation efficiency of 97.4% for Rhodamine B (RhB) within 6 min, which is 3.63 and 1.71 times that of BWO and CABB, respectively, and showed good stability in 5 cycle experiments. 3BWO/CABB can also effectively degrade norfloxacin (NOR) (60 min, 63.1%) and rifampicin (RFP) (60 min, 93.21%). Radical trapping experiments showed that -OH and -O2- play a major role in photodegradation. The excellent photocatalytic performance of BWO/CABB is primarily due to the strong interaction between CABB nanosheets and BWO nanoflowers, the tight interfacial contact and the Z-type heterostructure formed by good Fermi level (EF) alignment, which enhances the visible light absorption and inhibits the recombination of carriers. The study of double perovskite BWO/CABB Z-type heterojunction can provide a reference for developing high-performance catalysts.

Direct z-scheme ZnCo2S4/MOF-199 constructed by bimetallic sulfide modified MOF for photocatalytic hydrogen evolution

This work reports a simple ultrasonic impregnation method, using MOF-199 as the carrier platform to construct direct Z-Scheme ZnCo2S4/MOF-199 (ZCS/M) for stable and efficient photocatalytic hydrogen (H2) evolution. Under simulated sunlight, ZCS/M shows high photocatalytic activity, with H2 evolution amount of 11.6 mmol·g−1·h−1, which is 48.4 and 83.3 times of MOF-199 and ZCS respectively, and the apparent quantum efficiency (AQY) reaches 4.92 % at 420 nm. The successful construction of direct Z-Scheme heterojunction greatly improves the separation of electron-hole pairs to promote photocatalytic H2 evolution. In addition, ZCS is evenly anchored on the surface of MOF-199, which effectively solves the problem that ZCS is easy to agglomerate, and exposes more active sites. In-situ XPS, VB, band gap, work function and Fermi level is used to analyze the Fermi level alignment, interfacial built-in electric field and band bending, and photocatalysis mechanism and path of electron migration is speculated. This work provides a new strategy for the design and construction of direct Z-Scheme heterojunction with transition metal sulfide on the platform of MOF to enhance photocatalytic H2 evolution during water splitting.

Tuning interfacial charge transfer for efficient visible-light-driven photodegradation and simultaneous H2 evolution

The edge-graphitized carbon nitride (C3N4Cg) was prepared by secondary pyrolysis to construct ZnO/C3N4Cg (ZCN) type-Ⅱ heterojunction photocatalyst via a facile sonication dispersion method, which achieved ∼7.04-fold and ∼18.3-fold enhanced visible-light-driven photocatalytic performance for refractory micropollutant removal and simultaneous hydrogen (H2) evolution respectively compared to conventional ZnO/g-C3N4 Step-scheme heterojunction. The apparent quantum efficiency of the ZCN0.4 heterojunction reaches 0.92% (λ = 420 nm). Such excellent performance stems from that the edge-graphene moieties stitched onto the interface of heterojunction extend light absorption to the full visible spectrum, meanwhile, the built-in electric field generated during Fermi level alignment accompanying favorable band-bending structure provides an effective pathway for the rapid migration of photoinduced electrons via the edge graphene channel to improve interfacial charge separation efficiency. Interestingly, the midgap states introduced in ZCN heterojunction could temporarily retain photoexcited electrons to effectively inhibit the in situ carrier recombination for improved photocatalytic H2 evolution. Moreover, ZCN/peroxymonosulfate system exhibited excellent anti-interference performance against complex water bodies under visible illumination due to the synergistic effect between the co-existing anions and organic matter. Meanwhile, the eco-friendly nature of the ZCN/peroxymonosulfate system showed no biotoxicity of reaction filtrate on cell proliferation after treatment, which avoided secondary contamination. Considering the outstanding performance in photocatalysis, the ZCN system exhibits broad potential for practical applications in water pollution control and green energy production.

Thermoelectric properties of self-assembled thiophene derivatives: effect of molecular structure on doping efficiency and Fermi level alignment

Organic materials have emerged as promising candidates for thermoelectric (TE) applications, offering distinct advantages over their inorganic counterparts. While polymers have received considerable attention in this context, small organic semiconductors have not been widely explored due to the challenge of obtaining high conducting films. Advancing this field necessitates the development of novel materials and a deeper understanding of their structure-dependent TE properties. The present study reports how slight modifications to the molecular structure affect the TE characteristics of two thiophene-based small molecules (OT1 and OT2), which differ only in their end substitution with varying acceptor strengths. This difference led to distinct morphologies when self-assembled. OT2 doped with FeCl3 displayed high electrical conductivity and power factor (52.0 μW/mK2), while OT1 exhibited significantly lower conductivity and power factor (1.6 μW/mK2) under identical conditions. The weaker acceptor strength of the end group in OT2 resulted in higher doping efficiency. Improved charge carrier transport in OT2 was due to a low-lying highest occupied molecular orbital (HOMO) energy level and enhanced density of states around the HOMO after doping. Maintaining its self-assembly after doping was also crucial for better electrical properties. These findings emphasize the importance of molecular engineering in developing high-performance TE materials.

In-situ growth of Cs2AgBiBr6 perovskite nanocrystals on Ti3C2Tx MXene nanosheets for enhanced photocatalytic activity

Photocatalytic degradation of organic dyes is an efficient, safe and environmentally-friendly technique for improving environmental purification. Exploring high-quality and low-cost photocatalysts for photocatalytic degradation with enhanced light harvesting capability and efficient charge separation and transport remains a challenge. Herein, we utilize the two-dimensional (2D) MXene, Ti3C2Tx nanosheets as a high efficiency co-catalyst to construct the Cs2AgBiBr6/Ti3C2Tx Schottky heterostructure by in-situ growth of Cs2AgBiBr6 perovskite NCs on Ti3C2Tx nanosheets via thermal injection method. The optimized Cs2AgBiBr6/Ti3C2Tx nanocomposites show a substantially enhanced organic dye degradation rate, which is ∼4.7 times higher than that of the pristine Cs2AgBiBr6 NCs. Such outstanding photocatalytic activity of Cs2AgBiBr6/Ti3C2Tx nanocomposites stems from the strong interaction between Cs2AgBiBr6 NCs and highly conductive 2D Ti3C2Tx nanosheets, a large interface contact area, and the favorable Fermi level alignment, which significantly promote the charge transfer and separation efficiency while suppressing the electron-hole recombination. More importantly, the Cs2AgBiBr6/MXenes–based photocatalysts exhibit good repeatability and long-term stability. It is demonstrated that this work will guide the construction of high-efficiency and stable photocatalytic materials for environmental purification.

Dual step-scheme heterojunction with full-visible-light-harvesting towards synergistic persulfate activation for enhanced photodegradation

The occurrence of micropollutants in aquatic media raises great concern because of their biological toxicity and persistence. Herein, visible-light-driven photocatalyst titanium dioxide/graphitic carbon nitride/triiron tetraoxide (TiO2-x/g-C3N4/Fe3O4, TCNF) with oxygen vacancies (Ov) was prepared via a facile hydrothermal-calcination method. The complementary visible-light co-absorption among semiconductors enhances light-harvesting efficiency. The built-in electric field formed during Fermi level alignment drives photoinduced electron transfer to improve charge separation across the interfaces. The increased light-harvesting and favorable energy band bending significantly enhance the photocatalytic performance. Therefore, TCNF-5-500/persulfate system could effectively photodegrade bis-phenol A within 20 min under visible-light irradiation. Moreover, the superior durability, non-selective oxidation, adaptability, and eco-friendliness of the system were confirmed by different reaction conditions and biotoxicity assessment. Furthermore, the photodegradation reaction mechanism was presented according to the major reactive oxygen species produced in the system. Thus, this study constructed a dual step-scheme heterojunction by tuning visible-light absorption and energy band structure to increase the charge transfer efficiency and photogenerated carrier lifetime, which has great potential for environmental remediation using visible photocatalysis.

Design and synthesis of multifaceted dicyanomethylene rhodanine linked thiophene: a SnOx–perovskite dual interface modifier facilitating enhanced device performance through improved Fermi level alignment, defect passivation and reduced energy loss

A new small molecule (AA6) is designed and synthesized for the first time and used as a dual interface modifier. The enhanced device performance observed after the interface modification is analyzed and discussed.

Enhancement of H2S sensing performance of rGO decorated CuO thin films: experimental and DFT studies

We demonstrate a highly selective and sensitive Cupric oxide (CuO) thin film-based low concentration Hydrogen sulfide (H2S) sensor. The sensitivity was improved around three times by decorating with reduced graphene oxide (rGO) nanosheets. CuO thin films were deposited by Chemical Vapor Deposition followed by inter-digital electrode fabrication by a thermal evaporations system. The crystal structure of CuO was confirmed by x-ray diffraction. The sensing response of pristine CuO was found around 54% at 100 °C to 100 ppm of H2S. In contrast, the sensing response was enhanced to 167% by decorating with rGO of 1.5 mg ml−1 concentration solution. The sensing was improved due to the formation of heterojunctions between the rGO and CuO. The developed sensor was examined under various gas environments and found to be highly selective towards H2S gas. The improvement in sensing response has been attributed to increased hole concentration in CuO in the presence of rGO due to the Fermi level alignment and increased absorption of H2S molecules at the rGO/CuO heterojunction. Further, electronic structure calculations show the physisorption behavior of H2S molecules on the different adsorption sites. Detailed insight into the gas sensing mechanism is discussed based on experimental results and electronic structure calculations.

Metallic-phase of MoS2 as potential electrocatalyst for hydrogen production via water splitting: A brief review

Metallic polymorph of MoS 2 (1T-MoS 2 ) is an emerging and highly promising electrocatalyst owing to its high intrinsic conductivity, large interlayer spacing, hydrophilic surface, high specific surface area, faster in-plane charge transfer, and lower energy barrier for hydrogen adsorption and desorption. This article discusses a brief overview of electrocatalytic property evolutions of metallic 1T-MoS 2 for its application in electrocatalytic hydrogen production via water splitting. A summary is presented on various techniques that have been reported to date for developing the 1T-MoS 2 phase and their efficacy towards electrocatalytic hydrogen generation. This review also explains the thermodynamic parameter dependence of electrocatalytic enhancements of metallic MoS 2 with state-of-the-art theoretical reasoning and experimental details. The Gibbs free energy of adsorption/desorption of H 2 in 1T-MoS 2 is lower, and therefore outperforms electrocatalytic hydrogen production with respect to 2H–MoS 2 . The various engineering strategies, applied on the material, open avenues for systems devoted to water splitting. • Metallic polymorph (1T-MoS 2 ) of 2D-MoS 2 is an emerging and promising electrocatalyst for Hydrogen Evolution Reaction (HER). • 1T-MoS 2 exhibits lower Gibbs free energy of H 2 adsorption/desorption, thereby outperforms 2H–MoS 2 as electrocatalyst. • Various strategies are discussed for effective phase transition of 2D-MoS 2 from semiconducting (2H) to metallic (1T) MoS 2 . • Bandgap engineering, exposing active sites, and Fermi level alignment have shown extraordinary results for enhancing HER.

A critical review on prospects and challenges of metal-oxide embedded g-C3N4-based direct Z-scheme photocatalysts for water splitting and environmental remediation

Solar photocatalysis is one of the most emerging feasible solutions for the prevailing energy and environmental issues in a sustainable manner. In this context, graphitic carbon nitride (g-C3N4) is evolving as a highly appropriate and intriguing material due to its fascinating physicochemical features. Low absorption range, low specific surface area (SSA), fewer active sites, high electron-hole recombination rate, and insufficient redox potentials limits its photocatalytic performance. The widely reported strategies to address the above limitations include doping, morphological tuning, type II heterojunction, Z-scheme heterojunctions etc. Among these, in recent years, Direct Z-scheme based photocatalysts has emerged as the most promising one as it maximizes the redox potential and enables effective spatial charge separation. The widely reported g-C3N4-based direct Z-scheme systems includes g-C3N4-wide band gap SCs, g-C3N4-narrow band gap SCs and g-C3N4-based ternary composite. Keeping this in mind, the current review tries to identify the challenges and future prospects of g-C3N4-based direct Z-scheme systems for energy and environmental applications. This review reveals that the g-C3N4-narrow band gap SC system is advantages over the g-C3N4-wide band gap SC system for solar photocatalytic applications as it produces larger amount of photo-generated effective charge carriers. The review found that, ternary Z-scheme system can perform better, however minute optimization at molecular level is still required. Additionally, in depth studies on the Fermi level and associated Fermi level alignment are needed for better understanding of Z-scheme systems and for its commercialization.

Efficient Photon Extraction in Top-Emission Organic Light-Emitting Devices Based on Ampicillin Microstructures.

The desire to enhance the efficiency of organic light-emitting devices (OLEDs) has driven to the investigation of advanced materials with fascinating properties. In this work, the efficiency of top-emission OLEDs (TEOLEDs) is enhanced by introducing ampicillin microstructures (Amp-MSs) with dual phases (α-/β-phase) that induce photoluminescence (PL) and electroluminescence (EL). Moreover, Amp-MSs can adjust the charge balance by Fermi level (EF ) alignment, thereby decreasing the leakage current. The decrease in the wave-guided modes can enhance the light outcoupling through optical scattering. The resulting TEOLED demonstrates a record-high external quantum efficiency (EQE) (maximum: 68.7% and average: 63.4% at spectroradiometer; maximum: 44.8% and average: 42.6% at integrating sphere) with a wider color gamut (118%) owing to the redshift of the spectrum by J-aggregation. Deconvolution of the EL intensities is performed to clarify the contribution of Amp-MSs to the device EQE enhancement (optical scattering by Amp-MSs: 17.0%, PL by radiative energy transfer: 9.1%, and EL by J-aggregated excitons: 4.6%). The proposed TEOLED outperforms the existing frameworks in terms of device efficiency.

Magnetic Field Enhanced Electrocatalytic Oxygen Evolution of NiFe-LDH/Co3 O4 p-n Heterojunction Supported on Nickel Foam.

Here, a strategy to regulate the electron density distribution by integrating NiFe layered double hydroxides (NiFe-LDH) nanosheets with Co3 O4 nanowires to construct the NiFe-LDH/Co3 O4 p-n heterojunction supported on nickel foam (NiFe-LDH/Co3 O4 /NF) for electrocatalytic oxygen evolution reaction (OER) is proposed. The p-n heterojunction can induce the charge redistribution in the heterogeneous interface to reach Fermi level alignment, thus modifying the adsorption free energy of *OOH and improving the intrinsic activity of the catalyst. As a result, NiFe-LDH/Co3 O4 /NF exhibits outstanding OER performance with a low overpotential of 274mV at a current density of 50mAcm-2 and long-time stability over 90h. Moreover, NF can serve as a magnetic core that induces the exchange bias effect between the magnetic substrate and the active species under the action of the magnetic field, resulting in decreased magnetoresistance and weakened scattering of spin electrons, which further lowers the OER overpotential by 25mV @ 50mAcm-2 under a 10000G magnetic field. This work provides a new perspective on the design of p-n heterojunction catalysts and a deeper understanding of the magnetic field-enhanced electrocatalytic reactions.

Piezo-phototronic effect boosted catalysis in plasmonic bimetallic ZnO heterostructure with guided fermi level alignment

High photo-induced carriers (PCs) recombination rate, low PC mobility, and limited solar energy utilization rate are the three main roadblocks that severely limit the photocatalytic activity of semiconductors. In this work, we designed a plasmonic bimetallic ZnO nanorod array (Au/ZnO/Pt) with Au nanoparticles (NPs) located on top of ZnO nanorods and Pt NPs evenly distributed on the ZnO nanorods for improving catalysis through the piezo-phototronic effect. With the rational design of guided Fermi level alignment, the photoinduced hot electrons of Au NPs with localized surface plasmon resonance effect can transfer to Pt NPs through ZnO to promote the separation and migration of PC. More importantly, under the stimulation of ultrasound, ZnO with piezo-phototronic effect generates an interfacial piezo-potential, thereby further promoting the separation and transport of carriers in compliance with the direction of piezo-potential to promote the surface redox reaction. Under the synergy of piezo-phototronic effect and localized surface plasmon resonance effect, the Au/ZnO/Pt realized 97.5% dye degradation in 60 min, which was 1.2, 1.36, and 1.79 folds of that with Au/ZnO/Au, Pt/ZnO/Pt, and ZnO, respectively. The unique plasmonic bimetallic heterostructure with piezo-phototronic effect and guided Fermi level alignment can guide the directional migration of carriers and provides useful instruction for the design of high-efficiency catalysts.

Surface state-induced barrierless carrier injection in quantum dot electroluminescent devices

The past decade has witnessed remarkable progress in the device efficiency of quantum dot light-emitting diodes based on the framework of organic-inorganic hybrid device structure. The striking improvement notwithstanding, the following conundrum remains underexplored: state-of-the-art devices with seemingly unfavorable energy landscape exhibit barrierless hole injection initiated even at sub-band gap voltages. Here, we unravel that the cause of barrierless hole injection stems from the Fermi level alignment derived by the surface states. The reorganized energy landscape provides macroscopic electrostatic potential gain to promote hole injection to quantum dots. The energy level alignment surpasses the Coulombic attraction induced by a charge employed in quantum dots which adjust the local carrier injection barrier of opposite charges by a relatively small margin. Our finding elucidates how quantum dots accommodate barrierless carrier injection and paves the way to a generalized design principle for efficient electroluminescent devices employing nanocrystal emitters.

Nanoscale Probing of Surface Charges in Functional Copper‐Metal Organic Clusters by Kelvin Probe Force Microscopy for Field‐Effect Transistors

AbstractMetal–organic frameworks (MOFs) have recently attracted a great deal of attention especially as conceivable advanced gate dielectrics for next‐generation field‐effect transistors (FETs) and memory device applications. Dielectric surface charge retention mapping is essential for gauging the leakage current and threshold voltage stability. Due to a dearth of systematic real‐time surface charge probing of MOFs dielectrics, in this work, the nanoscale Kelvin probe force microscopy (KPFM) technique is employed for the contact potential difference (CPD) analysis of developed hybrid copper–metal–organic clusters (Cu‐MOCs)/Si systems. Films are synthesized through the sol‐gel process consisting of copper metal core linked with organic ligands. With 3V positive and negative DC bias, charge injection offset between positive and negative bias is observed to be ≈190 mV, indicating the intrinsic Fermi level alignment between KPFM tip and sample surface charges. The high‐resolution surface CPD mappings bring forth the white (≈98 mV) and black (≈−91 mV) contrast profiles with hole (5.09 × 1012 cm−2) and electron (4.74 × 1012 cm−2) charge densities. Also, the 17 h time‐lapse enables the holes and electrons CPD mapping diameter shrinkage by ≈32% and 46%, respectively. In furtherance, a conductive filament model for host‐guest proton‐assisted conduction with charge hopping through fixed/mobile ions is proposed.

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Fabrication and characterization of Cesium-doped Tungstate nanorods for Near-Infrared light absorption in dye sensitized solar cells

Near-infrared (NIR) light-absorbing materials have been widely used in vehicle and building applications due to their transparent film structure with a high absorption rate in the NIR range and a high transmittance rate for visible light. In this paper, Cesium-doped Tungstate (Cs0.33WO3) has been combined with dye-sensitized solar cell (DSSC) to convert the heat energy (absorbed from NIR light) into photocurrent. Cs0.33WO3 Nanorods were synthesized and embedded in the active layer of the cell. The mesoporous Titanium dioxide (TiO2) on FTO has also been used as the electron transport layer (ETL). This simple structure of FTO/TiO2/Cs0.33WO3 resulted in a poor Fermi level alignment and a low voltage. However, a buffer layer made of Poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-DOF) improved the Fermi level alignment and the quality of the interface in the modified structure of FTO/TiO2/PFN-DOF/Cs0.33WO3 which improved the initial Jsc from 0.38 μA/cm2 to 0.45 μA/cm2 for the NIR range. In addition, the Jsc of this modified structure improved from 18.84 μA/cm2 to 27.56 μA/cm2 for the visible light. Finally, by using a P25-doped TiO2 layer, the Jsc increased from 0.70 μA/cm2 to 31.12 μA/cm2 for both NIR and visible light, respectively.