Graphene Field Effect Transistors with Mica as Gate Dielectric Layers

Chemical vapor deposited monolayer graphene is transferred onto atomically flat and ultra‐thin muscovite mica to study the transport characteristics of graphene with a test structure of mica‐based graphene field effect transistor (GFET). The transfer curve of the 24 nm mica‐based GFET shows an effective carrier mobility of 2748 cm²/Vs and a transconductance of 3.36 μS, a factor of 2 and 7 larger than those values obtained from 40 nm SiO₂ based GFET, respectively. The results demonstrate that mica is an excellent gate dielectric material due to its high dielectric constant, high dielectric strength, and atomically flat surface.     

Thickness scaling and electric properties of highly (111) oriented Nb-doped Pb(Zr,Ti)O<Subscript>3</Subscript> thin film prepared at low temperature by a sol–gel route

A series of highly (111) oriented Pb(Nb_0.01Zr_0.2Ti_0.8)O₃ (PNZT) thin films of variant thickness were successfully achieved on Pt/Ti/SiO₂/Si substrate by a sol–gel route. By introducing Pb_0.8Ca_0.1La_0.1Ti_0.975O₃ (PLCT) layer between the PNZT film and Pt electrode, the PNZT film could be crystallized at as low as 500 °C. When a maximum applied voltage is 3 V, it was found that the PNZT film with PLCT seed layer possessed higher remnant polarization (22 μC/cm²) as film thickness was scaled down to 50 nm. It was also found that enhanced pyroelectric properties could be observed in 50-nm thickness PNZT thin film due to its relatively low dielectric constant. The results demonstrated that the film thickness could be scaled down for low voltage operations using lattice matched interface between PNZT film and PLCT seed layer on Pt/Ti/SiO₂/Si substrate, and this interface optimization is the key technology for synthesizing thin PNZT films at low temperature with good insulating and electric properties.     

Mechanical Exfoliation of Select MAX Phases and Mo4Ce4Al7C3 Single Crystals to Produce MAXenes

MXenes—2D carbides/nitrides derived from their bulk nanolamellar M_n+1AX_n phase (MAX) counterparts—are, for the most part, obtained by chemical etching. Despite the fact that the M? A bonds in the MAX phases are not weak, in this work it is demonstrated that relatively large MAX single crystals can be mechanically exfoliated using the adhesive tape method to produce flakes whose thickness can be reduced down to half a unit cell. The exfoliated flakes, transferred onto SiO₂/Si substrates, are analyzed using electric force microscopy (EFM). No appreciable variation in EFM signal with flake thickness is found. EFM contrast between the flakes and SiO₂ not only depends on the contact surface potential, but also on the local capacitance. The contribution of the latter can be used to show the metallic character—confirmed by four‐contact resistivity measurements—of even the thinnest of flakes. Because the A‐layers are preserved, strictly speaking MXenes are not dealt with in this work, but rather MAXenes. This is important in the case where the “A” layers contain magnetic elements such as Mo₄Ce₄Al₇C₃, whose structure is a derivative of the MAX structure.     

Van der Waals Epitaxial Growth of 2D Metallic Vanadium Diselenide Single Crystals and their Extra‐High Electrical Conductivity

2D metallic transition‐metal dichalcogenides (MTMDs) have recently emerged as a new class of materials for the engineering of novel electronic phases, 2D superconductors, magnets, as well as novel electronic applications. However, the mechanical exfoliation route is predominantly used to obtain such metallic 2D flakes, but the batch production remains challenging. Herein, the van der Waals epitaxial growth of monocrystalline, 1T‐phase, few‐layer metallic VSe₂ nanosheets on an atomically flat mica substrate via a “one‐step” chemical vapor deposition method is reported. The thickness of the VSe₂ nanosheets is precisely tuned from several nanometers to several tenths of nanometers. More significantly, the 2D VSe₂ single crystals are found to present an excellent metallic feature, as evidenced by the extra‐high electrical conductivity of up to 10⁶ S m^?1, 1–4 orders of magnitude higher than that of various conductive 2D materials. The thickness‐dependent charge‐density‐wave phase transitions are also examined through low‐temperature transport measurements, which reveal that the synthesized 2D metallic 1T‐VSe₂ nanosheets should serve as good research platforms for the detecting novel many‐body states. These results open a new path for the synthesis and property investigations of nanoscale‐thickness 2D MTMDs crystals.     

MoS<Subscript>2</Subscript>/h-BN heterostructures: controlling MoS<Subscript>2</Subscript> crystal morphology by chemical vapor deposition

Tuning the properties of van der Waals heterostructures based on alternating layers of two-dimensional materials is an emerging field of research with implications for electronics and photonics. Hexagonal boron nitride (h-BN) is an attractive insulating substrate for two-dimensional materials as it may exert less influence on the layer’s properties than silica. In this work, MoS₂ layers were deposited by chemical vapor deposition (CVD) on thick h-BN flakes mechanically exfoliated deposited on Si/SiO₂ substrates. CVD affords the controllable, large-scale preparation of MoS₂ on h-BN alleviating shortcomings of manual mechanical assembly of such heterostructures. Electron microscopy revealed that in-plane and vertical to the substrate MoS₂ layers were grown at high yield, depending on the sample preparation conditions. Raman and photoluminescence spectroscopy were employed to assess the optical and electronic quality of MoS₂ grown on h-BN as well as the interactions between MoS₂ and the supporting substrate. Compared to silica, MoS₂ layers grown on h-BN are less prone to oxidation and are subjected to considerably weaker electronic perturbation.     

Thick films of monocrystalline silicon on SiO2 obtained via embedding SiO2 stripes in bulk silicon

A method is presented for obtaining thick films of single-crystal Si on SiO₂ stripes patterned on bulk silicon. We use a classical deposition of a 0.5 μ m poly-Si coating on patterned stripes of SiO₂ grown on 4″ wafers. A controlled sinking of the SiO₂ stripes into the bulk is obtained, via melting the top surface of the substrate. The thick films of single-crystal Si obtained on SiO₂ (typically 20–40 μm in thickness) are free of defects and exhibit the same surface roughness as before crystallization.     

Structural and dielectric properties of Ba0.7Sr0.3TiO3 thin films grown on thin Bi layer-coated Pt(111)/Ti/SiO2/Si substrates

In this work, the growth and study of dielectric properties of Ba_0.7Sr_0.3TiO₃ (BST) thin films grown on thin Bi layer coated Pt(111)/Ti/SiO₂/Si substrates, depending on thin Bi layer thickness is reported. The BST thin film (thickness 180 nm) grown on 10-nm-thick Bi layer exhibited more improved structural and dielectric properties than that grown on bare Pt(111)/Ti/SiO₂/Si substrate. The 10-nm-thick Bi layer in optimum configuration was effective for the grain growth of BST phase and suppressed the formation of the oxygen-deficient layer at the interface between the BST thin film and bottom electrode, which resulted in an increase in dielectric constant and a decrease in leakage current density of the Pt/BST thin film/Pt capacitor.     

Improved growth control of atomically thin WSe2 flakes using pre-deposited W source

The improvement in the growth yield and control of atomically thin WSe₂ flakes by chemical vapor deposition (CVD) using pre-deposited WO₃ nanopowders as a W source is demonstrated. WO₃ nanopowders are pre-deposited on the growth substrate and utilized as a W source instead of separate W sources in the CVD system. In this way, mostly mono or bilayer WSe₂ flakes are grown on the growth substrate with high density and an average size of around 20 μm. The devices based on the as-grown WSe₂ flakes show p-type behaviors with a high on/off ratio of ~?10⁵ and carrier mobility of ~?0.5 cm² V^??1 s^?1 as well as a large positive photoresponse. The density and size of WSe₂ flakes can be controlled by adjusting the amount of pre-deposited WO₃ nanopowders. This approach can be used to grow W-based two-dimensional materials as well as their heterostructures with other materials such as graphene and carbon nanotubes.

     

“Snowing” Graphene using Microwave Ovens

Developing a simple and industrially scalable method to produce graphene with high quality and low cost will determine graphene's future. The two conventional approaches, chemical vapor deposition and liquid‐phase exfoliation, require either costly substrates with limited production rate or complicated post treatment with limited quality, astricting their development. Herein, an extremely simple process is presented for synthesizing high quality graphene at low‐cost in the gas phase, similar to “snowing,” which is catalyst‐free, substrate‐free, and scalable. This is achieved by utilizing corona discharge of SiO₂/Si in an ordinary household microwave oven at ambient pressure. High quality graphene flakes can “snow” on any substrate, with thin‐flakes even down to the monolayer. In particular, a high yield of ≈6.28% or a rate of up to ≈0.11 g h^?1 can be achieved in a conventional microwave oven. It is demonstrated that the snowing process produces foam‐like, fluffy, 3D macroscopic architectures, which are further used in strain sensors for achieving high sensitivity (average gauge factor ≈ 171.06) and large workable strain range (0%–110%) simultaneously. It is foreseen that this facile and scalable strategy can be extended for “snowing” other functional 2D materials, benefiting their low‐cost production and wide applications.     

Studies of BaTiO3 thin films on different bottom electrode

BaTiO₃ (BT) thin films were prepared on Pt/Ti/SiO₂/Si and Ru/Ti/SiO₂/Si substrates by a modified sol-gel technique. The microstructure of the films was characterized by atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy. The results showed that BT thin films crystallized with perovskite structure. Compared to BT film on Pt/Ti/SiO₂/Si substrate, BT thin film deposited on Ru electrode has similar dielectric constant, while it has higher dielectric loss. C–E curve for BT film on Pt/Ti/SiO₂/Si was more symmetrical around zero-bias field than C–E curve for BT film on Ru/Ti/SiO₂/Si substrate. The tunability was 52.02% for BT film on Pt electrode, which was 33.42% on Ru electrode, at 275 kV/cm and room temperature. The leakage current density of BT on Pt electrode was about an order of magnitude lower than BT film on Ru electrode at the applied electrical field below 150 kV/cm. The leakage conduction mechanism was investigated.     

Edge‐Epitaxial Growth of InSe Nanowires toward High‐Performance Photodetectors

Semiconducting nanowires offer many opportunities for electronic and optoelectronic device applications due to their unique geometries and physical properties. However, it is challenging to synthesize semiconducting nanowires directly on a SiO₂/Si substrate due to lattice mismatch. Here, a catalysis‐free approach is developed to achieve direct synthesis of long and straight InSe nanowires on SiO₂/Si substrates through edge‐homoepitaxial growth. Parallel InSe nanowires are achieved further on SiO₂/Si substrates through controlling growth conditions. The underlying growth mechanism is attributed to a selenium self‐driven vapor–liquid–solid process, which is distinct from the conventional metal‐catalytic vapor–liquid–solid method widely used for growing Si and III–V nanowires. Furthermore, it is demonstrated that the as‐grown InSe nanowire‐based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of 271 A W^?1, ultrahigh detectivity of 1.57 × 10¹⁴ Jones, and a fast response speed of microsecond scale. The excellent performance of the photodetector indicates that as‐grown InSe nanowires are promising in future optoelectronic applications. More importantly, the proposed edge‐homoepitaxial approach may open up a novel avenue for direct synthesis of semiconducting nanowire arrays on SiO₂/Si substrates.     

Growing Oxide Nanowires and Nanowire Networks by Solid State Contact Diffusion into Solution‐Processed Thin Films

New techniques to directly grow metal oxide nanowire networks without the need for initial nanoparticle seed deposition or postsynthesis nanowire casting will bridge the gap between bottom‐up formation and top‐down processing for many electronic, photonic, energy storage, and conversion technologies. Whether etched top‐down, or grown from catalyst nanoparticles bottom‐up, nanowire growth relies on heterogeneous material seeds. Converting surface oxide films, ubiquitous in the microelectronics industry, to nanowires and nanowire networks by the incorporation of extra species through interdiffusion can provide an alternative deposition method. It is shown that solution‐processed thin films of oxides can be converted and recrystallized into nanowires and networks of nanowires by solid‐state interdiffusion of ionic species from a mechanically contacted donor substrate. NaVO₃ nanowire networks on smooth Si/SiO₂ and granular fluorine‐doped tin oxide surfaces can be formed by low‐temperature annealing of a Na diffusion species‐containing donor glass to a solution‐processed V₂O₅ thin film, where recrystallization drives nanowire growth according to the crystal habit of the new oxide phase. This technique illustrates a new method for the direct formation of complex metal oxide nanowires on technologically relevant substrates, from smooth semiconductors, to transparent conducting materials and interdigitated device structures.     

Agglomeration of amorphous silicon film with high energy density excimer laser irradiation

In this paper, agglomeration phenomena of amorphous Si (α-Si) films due to high energy density excimer laser irradiation are systematically investigated. The agglomeration, which creates holes or breaks the continuous Si film up into spherical beads, is a type of serious damage. Therefore, it determines an upper energy limit for excimer laser crystallization. It is speculated that the agglomeration is caused by the boiling of molten Si. During this process, outbursts of heterogeneously nucleated vapor bubbles are promoted by the poor wetting property of molten silicon on the SiO₂ layer underneath. The onset of the agglomeration is defined by extrapolating the hole density as a function of the energy density of the laser pulse. A SiO₂ capping layer (CL) is introduced on top of the α-Si film to investigate its influence on the agglomeration. It is found that effects of the CL depend on its thickness. The CL with a thickness less than 300 nm can be used to suppress the agglomeration. A thin CL acts as a confining layer and puts a constraint on bubble burst, and hence suppresses the agglomeration.     

Electrochemically Exfoliated High‐Quality 2H‐MoS2 for Multiflake Thin Film Flexible Biosensors

2D molybdenum disulfide (MoS₂) gives a new inspiration for the field of nanoelectronics, photovoltaics, and sensorics. However, the most common processing technology, e.g., liquid‐phase based scalable exfoliation used for device fabrication, leads to the number of shortcomings that impede their large area production and integration. Major challenges are associated with the small size and low concentration of MoS₂ flakes, as well as insufficient control over their physical properties, e.g., internal heterogeneity of the metallic and semiconducting phases. Here it is demonstrated that large semiconducting MoS₂ sheets (with dimensions up to 50 µm) can be obtained by a facile cathodic exfoliation approach in nonaqueous electrolyte. The synthetic process avoids surface oxidation thus preserving the MoS₂ sheets with intact crystalline structure. It is further demonstrated at the proof‐of‐concept level, a solution‐processed large area (60 × 60 µm) flexible Ebola biosensor, based on a MoS₂ thin film (6 µm thickness) fabricated via restacking of the multiple flakes on the polyimide substrate. The experimental results reveal a low detection limit (in femtomolar–picomolar range) of the fabricated sensor devices. The presented exfoliation method opens up new opportunities for fabrication of large arrays of multifunctional biomedical devices based on novel 2D materials.     

Probing Effective Out‐of‐Plane Piezoelectricity in van der Waals Layered Materials Induced by Flexoelectricity

Many van der Waals layered 2D materials, such as h‐BN, transition metal dichalcogenides (TMDs), and group‐III monochalcogenides, have been predicted to possess piezoelectric and mechanically flexible natures, which greatly motivates potential applications in piezotronic devices and nanogenerators. However, only intrinsic in‐plane piezoelectricity exists in these 2D materials and the piezoelectric effect is confined in odd‐layers of TMDs. The present work is intent on combining the free‐standing design and piezoresponse force microscopy techniques to obtain and directly quantify the effective out‐of‐plane electromechanical coupling induced by strain gradient on atomically thin MoS₂ and InSe flakes. Conspicuous piezoresponse and the measured piezoelectric coefficient with respect to the number of layers or thickness are systematically illustrated for both MoS₂ and InSe flakes. Note that the promising effective piezoelectric coefficient (d^eff₃₃) of about 21.9 pm V^?1 is observed on few‐layered InSe. The out‐of‐plane piezoresponse arises from the net dipole moment along the normal direction of the curvature membrane induced by strain gradient. This work not only provides a feasible and flexible method to acquire and quantify the out‐of‐plane electromechanical coupling on van der Waals layered materials, but also paves the way to understand and tune the flexoelectric effect of 2D systems.     

Selective Formation of Titanium Silicide by Chemical Vapor Deposition Using Titanium Halides and Silicon Wafer as the Precursors

Titanium silicide thin films were formed on Si substrate by reaction of TiX ₄ (X=C1, Br) with Si under different experimental conditions. The Si consumption and titanium silicide obtained were calculated by the film thickness. In some reactions, titanium silicide thin film was found not only on the Si substrates but also on the SiO₂ wall at the outlet of the reaction chamber. The quantity of Si consumption and the quantity of silicon-containing materials obtained on the wall of the deposition chamber varied as the reaction conditions changed. The minimum Si consumption and maximum titanium silicide obtained on silicon were the most favorable result, found in the reaction of TiBr₄ with Si at 1000°C. The metallization reactions were studied in detail and the reaction pathway is proposed.     

Blue-shifted stimulated emission from ZnO films deposited on SiO2 by atomic layer deposition

ZnO films were prepared by atomic layer deposition upon a SiO₂ layer on a Si substrate and treated by rapid thermal annealing. The optically-pumped random lasing actions with low threshold values were observed in the ZnO films on SiO₂/Si substrates. With the decrease in ZnO film thickness or the increase in post-annealing duration, the stimulated emission shifted toward the shorter wavelength and the lasing threshold increased. The results can be attributed to the inter-diffusion between ZnO and SiO₂, which causes the modification of bandgap renormalization in ZnO.     

High‐Contrast SEM Imaging of Supported Few‐Layer Graphene for Differentiating Distinct Layers and Resolving Fine Features: There is Plenty of Room at the Bottom

For supported graphene, reliable differentiation and clear visualization of distinct graphene layers and fine features such as wrinkles are essential for revealing the structure–property relationships for graphene and graphene‐based devices. Scanning electron microscopy (SEM) has been frequently used for this purpose where high‐quality image contrast is critical. However, it is surprising that the effect of key imaging parameters on the image contrast has been seriously undermined by the graphene community. Here, superior image contrast of secondary electron (SE) images for few‐layer graphene supported on SiC and SiO₂/Si is realized through simultaneously tuning two key parameters—acceleration voltage (V_acc) and working distance (WD). The overlooked role of WD in characterizing graphene is highlighted and clearly demonstrated. A unified model of V_acc and WD dependence of three types of SE collected by the standard side‐attached Everhart‐Thornley (E‐T) SE detector is conceptually developed for mechanistically understanding the improved mass thickness contrast for supported few‐layer graphene. The findings reported here will have important implications for effective characterizations of atomically thick 2D materials and devices.     

Analysis of viscosity sensitivity for liquid property detection applications based on SAW sensors

An investigation of viscosity sensitivity for liquid property detection applications based on the ZnO/SiO₂/Si layered structure Love mode surface acoustic wave (SAW) sensors is presented. One of our interests in this paper is to optimize the SAW viscosity sensor under the condition of temperature stability by considering the relations among electromechanical coupling coefficient, viscosity sensitivity and temperature coefficient of delay (TCD). Some important results have been obtained by solving the system of coupled electromechanical field equations and Navier–Stokes equation. It is found that the electromechanical coupling coefficient and viscosity sensitivity can be further improved by adjusting the thickness of SiO₂ thin film and a zero TCD device also can be obtained by introducing a SiO₂ thin film with proper thickness. We try to obtain a device which possesses the viscosity sensitivity as high as possible and has zero TCD. Another interest of this paper is to improve the traditional viscosity sensitivity expression by considering the coupling effect between the liquid viscosity and density. It is shown that the coupling effect cannot be neglected from the numerical results. This modification could make the obtained viscosity more accurate. This analysis is meaningful for the manufactures and applications of the ZnO/SiO₂/Si structure Love wave sensor for liquid property detection.     

Measurement of the interlayer between aluminum and silicon dioxide using ellipsometric,capacitance-voltage and auger electron spectroscopy techniques

Ellipsometric and capacitance-voltage measurements were combined to detect both the AlSiO₂ interlayer and the SiSiO₂ interlayer for the Si/SiO₂/Al system. The AlSiO₂ interlayer was characterized by Auger electron spectroscopy (AES), combined with argon ion sputter profiling, of the Al/SiO₂/Si structure and also of the remaining SiO₂/Si structure after the aluminum had been chemically removed. An effective interlayer thickness is defined as the product of the interlayer thickness and the fractional change in the dielectric SiO₂ constant. The results of these experiments indicate that the Alz.sbnd;SiO₂ effective interlayer thickness has a range of 0.1–0.5 nm. The AES data can be readily interpreted if it is assumed that collision cascade mixing and recoil implantation occur as a consequence of sputter depth profiling through the aluminum.