Layer-dependent Bandgap Research Articles

Janus Doping of Sulfur into Platinum Diselenide Ribbons.

2D platinum diselenide (PtSe2), a novel member of the transition metal dichalcogenides (TMDCs) family, possesses many excellent properties, including a layer-dependent bandgap, high carrier mobility, and broadband response, making it promise for applications in technologies like field-effect transistors and room-temperature photodetectors. Doping represents an effective method to modify the electrical properties of 2D TMDCs and to bestow upon them additional functions. However, to date, little research has been conducted on the successful doping of 2D PtSe2 for modification. In this study, sulfur (S) powder is utilized during the chemical vapor deposition growth process of 2D PtSe2 ribbons and successfully integrated into the PtSe2 lattice through substitutional doping. The Au substrate significantly decreases the substitution energy of Se atoms in the lower layer of PtSe2, resulting in the formation of the Janus PtSSe structure. S-doped PtSe2 ribbons demonstrate significant symmetry breaking and enhanced electrical properties, showcasing a strong nonlinear optical response and certain synaptic plasticity, further simulating some neuromorphological processes. This study not only demonstrates a viable method for controllable doping and modification of 2D PtSe2 but also establishes a platform for exploring the characteristics of Janus TMDCs.

Solvent effects on the electrochemical performance of few layered MoS2 electrodes fabricated using FTO substrates

Owing to its exceptional structural, electrical, and optical features, Molybdenum disulphide (MoS2), a two-dimensional (2D) layered material with tuneable bandgap, finds its application in electrochemical supercapacitors for superior energy and power density. Because of their low toxicity and long-term energy storage, the development of MoS2-based supercapacitors is inevitable. The study of solvent effects on the electrochemical performance of a few layered MoS2 using FTO substrates is done for the first time to the best of our knowledge. Exfoliating bulk MoS2 powder in different solvents with variable surface tensions such as Ethanol, Ethylene Glycol (EG), Dimethylformamide (DMF), and Dimethyl Sulfoxide (DMSO) results in the formation of few-layered MoS2 structures. The sample’s structural, optical, and electrochemical behaviours are investigated using x-ray diffraction (XRD), atomic force microscopy (AFM), UV spectroscopy, Fourier transform infrared (FTIR), cyclic-voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). XRD confirms the formation of a 2D MoS2 film with (002) planes and the optical investigation revealed the variation of layer-dependent bandgap with solvents. We observe both faradaic and non-faradaic charge storage mechanisms in the samples and demonstrate a superior pseudocapacitive behaviour for MoS2 in DMF with a maximum specific capacitance of 34.25 F g−1 at a current density of 1 A/g.

Abnormal Thickness-Dependent Thermal Transport in Suspended 2D PdSe2.

Research on 2D materials originally focused on the highly symmetrical materials like graphene, h-BN. Recently, 2D materials with low-symmetry lattice such as PdSe2 have drawn extensive attention, due to the interesting layer-dependent bandgap, promising mechanical properties and excellent thermoelectric performance, etc. In this work, the phonon thermal transport is studied in PdSe2 with a pentagonal fold structure. The thermal conductivity of PdSe2 flakes with different thicknesses ranging from few nanometers to several tens of nanometers is measured through the thermal bridge method, where the thermal conductivity increases from 5.04W mk-1 for 60nm PdSe2 to 34.51W mk-1 for the few-layer one. The atomistic modelings uncover that with the thickness thinning down, the lattice of PdSe2 becomes contracted and the phonon group velocity is enhanced, leading to the abnormal increase in the thermal conductivity. And the upshift of the optical phonon modes contributes to the increase of the thermal conductivity as well by creating less acoustic phonon scattering as the thickness reduces. This study probes the interesting abnormal thickness-dependent thermal transport in 2D materials, which promotes the potential thermal management at nanoscale.

Heteroepitaxial Growth of Black Phosphorus on Tin Monosulfide.

Black phosphorus (Black P), a layered semiconductor with a layer-dependent bandgap and high carrier mobility, is a promising candidate for next-generation electronics and optoelectronics. However, the synthesis of large-area, layer-precise, single crystalline Black P films remains a challenge due to their high nucleation energy. Here, we report the molecular beam heteroepitaxy of single crystalline Black P films on a tin monosulfide (SnS) buffer layer grown on Au(100). The layer-by-layer growth mode enables the preparation of monolayer to trilayer films, with band gaps that reflect layer-dependent quantum confinement.

Study of enhancing photocatalytic activity of solvothermal grown MoS2 nanocrystals under visible light irradiation by the influence of hydrogen peroxide

In this article, two-dimensional molybdenum sulfide has been synthesized by simple and conventional solvothermal methods and the combined effect of MoS2 nanocrystals and hydrogen peroxide in photocatalytic degradation of methylene blue (MB) in water has been investigated under visible light. Synthesized MoS2 nanocrystals have been characterised by FEG-TEM, FE-SEM, XRD, Raman, FTIR, UV–Vis absorption, PL and TCSPC studies. FEG-TEM and FE-SEM images confirm nanosheets/nanoflowers like morphology and Raman & XRD patterns confirm the 2H-MoS2 phase. Layer-dependent absorption spectra and its Tauc plot were studied and the calculated band gap are found to be 1.73 eV and 1.43 eV for a few layers, bulk MoS2 respectively. This layer-dependent band gap energy is also verified by calculating electronic band structure for mono-layer, bi-layer and bulk MoS2 through density functional theory (DFT). The calculated average lifetime of in MoS2 is 1.5 ns. The kinetic study of the degradation of MB dye with MoS2 in the presence of H2O2 showed complete degradation of MB within 20 min. Whereas, pure MoS2 and H2O2 showed 28.27 % and 89.2 % degradation of MB within 210 min time intervals respectively. Thus, these results show MoS2 has good photocatalytic activity in the presence of H2O2.

Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions

Two-dimensional (2D) materials have gained significant attention owing to their exceptional electronic and optoelectronic properties, including high carrier mobility, strong light–matter interaction, layer-dependent band structure and band gap. The passivated surface of 2D materials enables the fabrication of van der Waals (vdW) heterojunctions by integrating them with various other materials, such as nanowires, nanosheets and bulk materials. Heterojunction photodetectors, specifically those composed of 2D materials and silicon (Si), have attracted considerable interest due to the well-established processing techniques associated with Si and the excellent performance of the related devices. The hybrid dimension vdW heterojunction composed of 2D materials and Si has the advantages of excellent performance, low fabrication cost, and easy integration with silicon-based devices. It has unique advantages in the field of heterojunction photodetectors. This review provides an overview of the recent advancements in photodetectors based on 2D material/Si heterojunctions. First, we present the background and motivation of the review. Next, we discuss the key performance metrics for evaluating photodetector performance. Then, we review the recent progress made in the field of 2D material/Si heterojunction photodetectors. Finally, we summarize the findings and offer future prospects.

Highly efficient and stable near-infrared photo sensor based on multilayer MoS2/p-Si integrated with plasmonic gold nanoparticles

The exceptional characteristics of two-dimensional materials make them highly efficient and stable for electronic and optoelectronic applications. These materials exhibit a range of beneficial properties, such as ultrafast carrier dynamics, layer-dependent energy bandgap, tunable optical properties, low power dissipation, high mobility, transparency, flexibility, simple fabrication, and ability to confine electromagnetic energy within extremely small volumes. In this work, infrared light (980 nm) photo sensors are fabricated based on a MoS2/p-Si substrate utilizing the plasmonic phenomenon of gold nanoparticles (AuNPs) to enrich the optoelectronic properties and to enhance the photoresponse. The infrared light response for (Au NPs MoS2) comes from the strong interlayer coupling, which narrow the energy gap in the heterojunction area, thus rendering heterostructures to longer wavelength detection ability. Considering the low light absorption due to indirect bandgap essence of multilayer MoS2, its infrared responsivity further enhanced by 100.21% with a response rate of 42.39/95.44 μs (1 kHz) at a bias of 3 V, a repeatability responsivity of up to 0.59 A/W, and a detectivity of 4.5 × 1010 Jones with a maximum of 9.57 mW/cm2 light intensity, which is maintained through surface plasmon resonance (SPR). The plasmon-assisted photo sensors can be seamlessly integrated into the semiconductor industry to boost the optoelectronic performance in practical applications.

Large-scale synthesis of two-dimensional indium telluride films for broadband photodetectors

2D III-VI semiconductors have emerged as promising materials for optoelectronic devices due to their tunable bandgaps, efficient light absorption and high carrier mobility. Among III-VI group, 2D indium telluride (InTe) has been studied very little compared with its well-known congeners such as InSe and GaSe. Although InTe possesses remarkable electrical and optical properties, the investigation of its device applications is greatly hindered due to the shortage of scalable synthesis method. Here, we synthesized centimeter-scale 2D InTe films via a pulsed laser deposition method. The structure of as-grown InTe films was systematically studied, exhibiting good continuity, uniformity and high degree of crystallinity. Meanwhile, layer-dependent bandgaps (1.21∼1.65 eV) were observed from the optical characterization. The InTe based photodetectors show a broadband photoresponse from ultraviolet (370 nm) to near-infrared region (980 nm). The photoresponsivity and detectivity of the InTe photodetectors can achieve 6.35 A/W and 1.55×1011 under 370 nm illumination, respectively, which outperform many photodetectors based on large-area 2D materials. Notably, InTe photodetectors also demonstrate strong layer-dependent photoresponse from 2 L to 10 L upon different wavelength illumination. Our work will inspire the research interests to further develop the practical applications of 2D InTe in the field of photodetection devices.

Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene

The emergence of phosphorene has generated significant interest in 2D group VA nanomaterials. Among this group, bismuthene exhibits layer-dependent direct bandgaps, high carrier mobility, and topological insulator properties because of its unique structure and ultrathin nature, distinguishing it as a promising candidate for photonic applications. Particularly, its outstanding stability in air makes bismuthene more advantageous than phosphorene for practical applications. Here, we provide a comprehensive review of recent advances regarding 2D bismuth by focusing on the aspects of methods of synthesis and photonic applications. First, the structure and fundamental properties of bismuthene are described, referring to its crystallinity and band structures, as well as to its nonlinear optical properties. Subsequently, the common synthesis methods for 2D bismuth are summarized, including both top-down and bottom-up approaches. Then, potential photonic applications based on 2D bismuth, involving nonlinear photonic devices, photocatalyst, and photodetectors, are illustrated. The performance, mechanisms, and features of the devices are discussed. Finally, the review is summarized and some challenges and future outlooks in this field are addressed.

Functionalized Ti3C2Tx MXene with layer-dependent band gap for flexible NIR photodetectors

Ti3C2Tx MXene as a representative material in the emerging two-dimensional (2D) MXene family with high conductivity, abundant functional surface terminals, and large layer spacing is supposed to show specific semiconducting properties like other 2D graphene or transition metal dichalcogenides, thus extending Moore's law beyond silicon. However, despite extensive efforts, the design of Ti3C2Tx MXene based semiconductor materials often depends on the availability of traditional semiconductors to form heterojunctions, where Ti3C2Tx MXene is still in metallic characters and is not in dominant status in the heterojunctions. Here, we demonstrate semiconducting Ti3C2Tx MXene modified with dodecyl (−C12H26) groups, as functionalized Ti3C2Tx MXene possesses opened and typical layer-dependent bandgap. The new arising characteristics, red-shift of characteristic peaks, and intensity ratio of the A1g(C)/A1g(Ti, C, Tx) in Raman spectroscopy suggested the successful grafting of the −C12H26 groups on the Ti3C2Tx MXenes. In addition, the theoretical calculations by density functional theory, photoluminescence spectrum, together with photoelectric measurements of Ti3C2Tx-C12H26 MXene on different layers, show a tunable bandgap of 0.49–2.15 eV and superior photoresponse properties in fabricating near infrared photodetectors.

Development of rGO based photodetector for visible light detection applications

An extremely high sensitivity for light detection is possible with a thin layer of graphene, graphene oxide and reduced graphene oxide on a light-absorbing substrate, such as silicon (Si). In the detection of weak light signals, graphene on Si devices can outperform traditional photo detectors. Due to their remarkable features, two-dimensional (2D) materials have attracted a lot of attention from the scientific community ever since the discovery of graphene. Dangling bonds are absent from 2D layered materials, which also feature optoelectronic characteristics and a layer-dependent tunable bandgap. Graphene oxide and reduced graphene oxide is synthesised in this paper and photo detection properties of reduced graphene oxide is explored for the wide optical range from 450 nm to 550 nm at 1 V and 30 μW intensity. Highest responsivity 0.14 A/W is obtained at 550 nm. The response time to achieve maximum photocurrent response is also obtained for the illuminated wavelengths of light.

Two-dimensional (2D) α-In2Se3/Ta2NiSe5 heterojunction photodetector with high sensitivity and fast response in a wide spectral range

As a typical representative of group III-VI two-dimensional (2D) layered semiconductors, α-In2Se3 with layer-dependent direct bandgap (1.39–1.45 eV) has attracted great attention in optoelectronics. At present, various α-In2Se3 based photodetectors have been developed, which show ultrahigh responsivity and detectivity at visible (VIS) wavelengths. However, despite the high sensitivity, the response time is quite long, and the realization of broadband detection from VIS to infra-red (IR) wavelengths is also challenging for these α-In2Se3 photodetectors. In this paper, we introduced a narrow and direct bandgap 2D semiconductor Ta2NiSe5 and made an α-In2Se3/Ta2NiSe5 heterojunction photodetector, which achieved a wide response wavelength of 405–1550 nm with high performance. Originating from the n-type nature of the two semiconductors and the particular type-I energy band alignment, the potential barriers of holes at the α-In2Se3/Ta2NiSe5 interface is much higher than that of electrons, which leads to an ultra-high responsivity. At 520 nm illumination, the photoresponsivity is 12 A/W and 533 A/W under a self-driven state and a slight bias voltage of 0.1 V, respectively. The highest detectivity is over 8.2 × 1013 Jones. Furthermore, the rise and fall time is shortly at 25 and 423 μs, respectively. We believe that the high-sensitive, broadband, and fast photodetector has great potential in the emerging field of 2D optoelectronics.

Rhombohedral Boron Monosulfide as a p-Type Semiconductor

Two-dimensional materials have wide ranging applications in electronic devices and catalysts owing to their unique properties. Boron-based compounds, which exhibit a polymorphic nature, are an attractive choice for developing boron-based two-dimensional materials. Among them, rhombohedral boron monosulfide (r-BS) has recently attracted considerable attention owing to its unique layered structure similar to that of transition metal dichalcogenides and a layer-dependent bandgap. However, experimental evidence that clarifies the charge carrier type in the r-BS semiconductor is lacking. In this study, we synthesized r-BS and evaluated its performance as a semiconductor by measuring the Seebeck coefficient and photo-electrochemical responses. The properties unique to p-type semiconductors were observed in both measurements, indicating that the synthesized r-BS is a p-type semiconductor. Moreover, a distinct Fano resonance was observed in Fourier transform infrared absorption spectroscopy, which was ascribed to the Fano resonance between the E(2) (TO) phonon mode and electrons in the band structures of r-BS, indicating that the p-type carrier was intrinsically doped in the synthesized r-BS. These results demonstrate the potential future application prospects of r-BS.

Layer Dependence of Thermally Induced Quantum Confinement and Higher Order Phonon Scattering for Thermal Transport in CVD-Grown Triangular MoS2

Heat dissipation and electron–phonon interaction hindering the charge carrier mobility are serious constraints for the fabrication of various integrated electronic and optoelectronic devices based on two-dimensional (2D) materials. In this paper, we examine the quantum confinement and phonon anharmonicity of different-layered MoS2, synthesized via the chemical vapor deposition technique. We explore the contribution of spin–orbit and interlayer couplings both theoretically (first-principles density functional theory) and experimentally (Raman and photoluminescence spectroscopies). Further, we demonstrate the thermally driven layer-dependent bandgap tunability in (1L, 3L, and 5L) triangular MoS2 grown over the SiO2/Si substrate. We have also scrutinized their phonon confinement behavior and thermal response in a low-temperature regime (80–300 K) using the optothermal Raman spectroscopy technique. A semiquantitative model comprising the volume and temperature effect provides insights into the nonlinear temperature-dependent phonon anharmonicity, revealing that the contribution of higher order (three) phonon scattering reduces with increasing layer numbers in MoS2. We further measure the interfacial thermal conductance (g) and thermal conductivity (ks) of synthesized MoS2, and the obtained values of g (and ks) are observed to increase (and decrease) with increasing layer number. Our study will advance the understanding of anharmonic behavior of phonons in different-layered MoS2 nanostructures for designing MoS2-based next-generation devices for various applications.

Chemistry of two-dimensional pnictogens: emerging post-graphene materials for advanced applications.

The layered allotropes of group 15 (P, As, Sb and Bi), also called two-dimensional (2D) pnictogens, have emerged as one of the most promising families of post-graphene 2D-materials. This is mainly due to the great variety of properties they exhibit, including layer-dependent bandgap, high charge-carrier mobility and current on/off ratios, strong spin-orbit coupling, wide allotropic diversity and pronounced chemical reactivity. These are key ingredients for exciting applications in (opto)electronics, heterogeneous catalysis, nanomedicine or energy storage and conversion, to name a few. However, there are still many challenges to overcome in order to fully understand their properties and bring them to real applications. As a matter of fact, due to their strong interlayer interactions, the mechanical exfoliation (top-down) of heavy pnictogens (Sb & Bi) is unsatisfactory, requiring the development of new methodologies for the isolation of single layers and the scalable production of high-quality flakes. Moreover, due to their pronounced chemical reactivity, it is necessary to develop passivation strategies, thus preventing environmental degradation, as in the case of bP, or controlling surface oxidation, with the corresponding modification of the interfacial and electronic properties. In this Feature Article we will discuss, among others, the most important contributions carried out in our group, including new liquid phase exfoliation (LPE) processes, bottom-up colloidal approaches, the preparation of intercalation compounds, innovative non-covalent and covalent functionalization protocols or novel concepts for potential applications in catalysis, electronics, photonics, biomedicine or energy storage and conversion. The past years have seen the birth of the chemistry of pnictogens at the nanoscale, and this review intends to highlight the importance of the chemical approach in the successful development of routes to synthesise, passivate, modify, or process these materials, paving the way for their use in applications of great societal impact.

Selective NO2 Detection by Black Phosphorus Gas Sensor Prepared via Aqueous Route for Ship Pollutant Monitoring

The emission of nitrogen dioxide (NO2) caused by marine transportation has attracted worldwide environmental concerns. Two-dimensional (2D) black phosphorus (BP) is an emerging semiconductive material with the advantages of high electron mobility, a layer-dependent direct band gap and a large specific surface area. These properties ensure excellent potential in gas-sensing applications. In this work, BP quantum dots (QDs) are synthesized from commercial red phosphorus (RP) fine powder via the aqueous route. The BP QDs show uniform size distribution with an average size of 2.2 nm. They are employed to fabricate thin film gas sensors by aerial-assisted chemical vapor deposition. The microstructure, morphology and chemical composition are determined by various characterizations. The sensor performances are evaluated with the optimized response set to 100 ppm NO2 of 10.19 and a sensitivity of 0.48 is obtained. The gas sensor also demonstrates excellent repeatability, selectivity and stability. The fabricated thin film gas sensor assembled by BP QDs exhibits prospective applications in selective NO2 detection for marine gaseous pollutant monitoring and control.

Progress in the synthesis of 2D black phosphorus beyond exfoliation

A considerable number of recent research have focused on two-dimensional (2D) black phosphorus (BP) since it was successfully prepared through mechanical exfoliation in 2014. After scaling down, BP with atomistic thickness shows fascinating semiconducting features with layer-dependent direct bandgap and high carrier mobility. The synthesis of high-quality few-layer BP thin films is critical to investigate their distinctive crystal structure, fundamental characteristics, as well as the potential applications in electronics, biomedicine, energy storage, photonics, and optoelectronics. Therefore, this review provides an overview of mono- and few-layer BP topic in the synthesis methods beyond exfoliation, including thinning treatments accompanied to exfoliation, conversion from red phosphorus to BP, and direct growth techniques. We summarize various attempts to control the BP sample's thickness and lateral dimensions during the synthesis. Furthermore, we discuss the current challenges and perspectives of large-scale growth of ultrathin BP which has been a bottleneck hindering wafer-scale device's development in this field. We hope to provide an insight into exploring some potential approaches practicable to synthesize high quality BP thin films utilized for developing high-performance nano-electronics and photonics, which may accelerate the progress of 2D BP toward real applications.

Graphene/PtSe2/Pyramid Si van Der Waals Schottky Junction for Room-Temperature Broadband Infrared Light Detection

Uncooled infrared (IR) photodetection has attracted increasing research interest due to its important applications in civil and military fields. Recently, platinum diselenide (PtSe2), a newly discovered group-10 two-dimensional (2D) noble metal dichalcogenide (NMD) member, has emerged as an attractive candidate for highly sensitive IR photodetection due to its layer-dependent bandgap transition from semiconductor to semimetal, wide optical absorption, and high carrier mobility. Here, we demonstrate the successful assembly of PtSe2/pyramid Si mixed-dimensional van der Waals (vdW) Schottky junction with a graphene transparent electrode. Due to the novel vertical device structure with a graphene top contact, the photodetector achieves an appealing device performance, including an ultrabroadband response up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10.6~\mu \text{m}$ </tex-math></inline-formula> , a high responsivity of 0.528 A/W, a large specific detectivity of ~1012 Jones, and a fast response time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.2~\mu \text{s}$ </tex-math></inline-formula> at zero bias voltage. More importantly, these findings have enabled the realization of an excellent room-temperature long-wave IR (LWIR) imaging capability and its utilization as an optical receiver in optical IR communication. Our work demonstrates a reliable approach to the construction of a high-performance Schottky junction device for room-temperature broadband IR photodetection.

Liquid crystal behaviors of micron and submicron-sized black phosphorus nanosheets

In this study, liquid crystal behaviors of black phosphorus (BP) nanosheets were investigated. A birefringence of approximately 10−4 was achieved for BP owing to its anisotropy, compared to that of 10−5 for graphene oxide. The submicron-sized BP nanosheets (S-BPNS) showed larger birefringence than the micron-sized BP nanosheets (M-BPNS) in blue–green light. The birefringence of both types increased with the incident wavelength; however, that of the S-BPNS remained a constant of approximately 3.8 × 10−4 near 672 nm, whereas that of the M-BPNS reached 4.8 × 10−4 before 753 nm. This inversion in red–yellow light was caused by the layer-dependent bandgap of BP. The anisotropy and tunable bandgap endow BP unique two-dimensional liquid crystal behaviors.

PEI/PEG functionalized Black Phosphorus prepared by a One-Pot method for a wide detection range CO2 gas sensor

Black Phosphorus (BP) has attracted considerable attention in gas detection fields attributing to layer-dependent direct bandgap, and high carrier mobility. Currently, many researchers use blended polyethyleneimine/polyethylene-glycol (PEI/PEG) functionalized two-dimensional (2D) material to detect carbon dioxide (CO2). However, the detection rage of CO2 sensor based blended PEI/PEG functionalized 2D materials still need further with widen to realize more application. In this paper, a RT (25 °C) CO2 sensor prepared by a One-Pot method based on a PEI/PEG functionalized BP composite material is developed, in which the existence of PEI/PEG onto BP was confirmed by SEM, EDS, etc. The gas-sensing experiment results show that the low limit of detection of PEI/PEG-BP gas sensor is 200 ppm CO2 under air conditions and a high limit of detection of 250,000 ppm CO2 under N2 conditions. Moreover, the PEI/PEG-BP sensor shows high selectivity, and excellent repeatability. The excellent gas-sensing properties of the PEI/PEG-BP sensor can be attributed to the meso-macropores structure, the recognition function of amino groups, and the formation of P-N heterojunction between BP and PEI. The reported results of PEI/PEG-BP sensor can provide a foundation for PEI/PEG functionalized 2D material and understand the sensing mechanism for compositing the 2D materials with polymer semiconductors.