Ultrathin Devices Research Articles

Raman Spectroscopy of 2D MoS2 Interacting with Metals

The research on molybdenum disulphide (MoS2) has progressed remarkably in the last decade, prompted by the increasing interest for this material as a potential candidate in future ultrathin optoelectronic devices. MoS2 is a layered semiconductor with a gap in the visible region, which can be exfoliated down to the mono-layer form. Since the discovery of the exceptional optoelectronic properties of 2D MoS2, Raman spectroscopy has been extensively used as a tool to characterize the structure and thickness of MoS2 films. Recent works on MoS2-metal interfaces have shown that Raman spectra are significantly affected by the interaction with metals. However, a complete understanding of how such interaction modifies the MoS2 vibrational properties is still lacking. Studying this subject with both experimental and theoretical methods will provide fundamental insight into the interface physics of MoS2-metal systems, which is crucial for the fabrication of metal contacts and for the development of metal-assisted synthesis methods. This review summarizes the main results concerning Raman spectroscopy studies of heterosystems between MoS2 and transition metals, providing both a basis and directions for future research.

Construction of Amphiphilic Indocyanine–Green–Based Langmuir Film and Drop–Casting Film with Photoelectric Conversion Properties

Molecular self–assembly is the automatic formation of functional assemblies of different structural components through weak, reversible, non–covalent interactions on the basis of molecular recognition. Amphiphilic molecules have a natural advantage in self–assembly at the gas/liquid interface. In this work, two amphiphilic molecules with a special molecular structure, indocyanine green (ICG) and a derivative of indocyanine green (CCS), were combined with two dye molecules (tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS) and nickel (II) phthalocyanine–tetrasulfonic acid tetrasodium salt (TsNiPc) for self–assembly through the Langmuir–Blodgett (LB) technique. The nanostructure and assembly behavior in ordered self–assembled films are effectively regulated by inducing dye molecules to form different types of aggregates (H– and J–aggregates). In addition, we prepared composite films containing the same functional components using the conventional drop–casting technique and performed a series of comparative experiments with LB films. The degree of hydrophilicity was found to be related to roughness, with LB composite films being flatter and denser, with the lowest roughness and the best hydrophobicity compared to drop–casting films. Notably, the LB films showed better optoelectronic properties under the same conditions, providing new clues for the application of optoelectronic functional ultrathin film devices.

Strain-enabled defect migration and defect activation in monolayer MoS2

Two-dimensional (2D) materials possess remarkable strain tolerance and exhibit strain-tunable properties, making them highly promising for flexible device applications. Defects within these materials significantly impact their optoelectronic response to strain. In this study, we investigate the influence of strain on the electrical properties of monolayer MoS2, emphasizing the pivotal role played by intrinsic defects in shaping the material’s electrical and optoelectronic response under strain. We observed an enhancement in photocurrent and persistent photoconductivity at specific strains, indicating the activation of defects at these strain values, thus enhancing the photoresponse. Moreover, our device exhibits diodic behavior at specific strain values after prolonged measurements under a static field, suggesting a reduction in the migration energy of defects caused by the applied strain. This finding holds significant implications for memory, logic, and flexible devices. Additionally, we observe an increase in electron mobility under tensile strain, with our flexible field-effect transistor exhibiting higher mobility (∼38 cm2 (V·s)−1) at 0.4% strain. Our study provides insight into the role of strain in the activation and migration of defects in monolayer MoS2 and opens up new avenues for the development of multifunctional ultra-thin flexible devices and memory applications.

Laser debonding application in ultra-thin device processing.

Laser debonding offers several advantages such as precision, speed, minimal damage, and being noncontact. We investigated the feasibility of utilizing laser processing technology in producing high-performance ultra-thin wafer devices at a low cost. We successfully utilized the 355nm ultraviolet nanosecond laser to develop a compatible laser debonding process for domestic temporary bonding adhesives, which effectively performed the laser lift-off of 8-in (20.3 cm) silicon/temporary bonding adhesive/glass substrate samples at a power density of 250m J/c m 2. We designed and developed a line light source shaping system that was ultimately able to produce a line spot with a length exceeding 1cm and an energy distribution unevenness of less than 10%.

Experimental Study on Critical Parameters Degradation of Nano PDSOI MOSFET under TDDB Stress.

In today's digital circuits, Si-based MOS devices have become the most widely used technology in medical, military, aerospace, and aviation due to their advantages of mature technology, high performance, and low cost. With the continuous integration of transistors, the characteristic size of MOSFETs is shrinking. Time-dependent dielectric electrical breakdown (TDDB) is still a key reliability problem of MOSFETs in recent years. Many researchers focus on the TDDB life of advanced devices and the mechanism of oxide damage, ignoring the impact of the TDDB effect on device parameters. Therefore, in this paper, the critical parameters of partially depleted silicon-on-insulator (PDSOI) under time-dependent dielectric electrical breakdown (TDDB) stress are studied. By applying the TDDB acceleration stress experiment, we obtained the degradation of the devices' critical parameters including transfer characteristic curves, threshold voltage, off-state leakage current, and the TDDB lifetime. The results show that TDDB acceleration stress will lead to the accumulation of negative charge in the gate oxide. The negative charge affects the electric field distribution. The transfer curves of the devices are positively shifted, as is the threshold voltage. Comparing the experimental data of I/O and Core devices, we can conclude that the ultra-thin gate oxide device's electrical characteristics are barely affected by the TDDB stress, while the opposite is true for a thick-gate oxide device.

Epitaxial growth of bilayer MoS2/MoSe2 heterostructure with high-responsivity enhanced by photogating effect

Van der Waals (vdW) semiconductors heterostructure based on transition metal dichalcogenides (TMDs) is a promising candidate for two-dimensional (2D) ultra-thin optoelectronic devices due to the strongly interlayer coupling and efficient carrier separation. However, the directly epitaxial growth of 2D TMDs vdW heterostructures with a clean interface and high photoresponsivity still remains a challenging. In this work, the epitaxial growth of bilayer MoS2/MoSe2 vdW heterostructure with atomic-layer clean interface is prepared via an iodine-assisted growth strategy. The electrical properties and optoelectronic characterization proves that the heterostructure device presents an enhanced photoresponsivity of 680 A/W, indicting the 21.25 times higher compared with pure MoS2, which benefited from the photogating effect formed by the localized electrons in this heterostructure. This study offers a well reference for the controlled growth of the TMDs vdW heterostructure, and the achieved heterostructure will prove promising applications in future integrated optoelectronic devices and systems.

Broadband plasmonic metasurface for spin-selective hologram in near-infrared band

Metasurface holography is a significant technology for the development of ultrathin optical devices. Multichannel holography has received close attention due to its applications in increasing information capacity and optical secret sharing. However, existing design methods for multichannel holography mainly focus on interleaved metasurface imaging in the vertical direction, which will inevitably suffer from the unexpected crosstalk affecting imaging quality. This work presents noninterleaved spin-selective metasurfaces by combining the geometric phase and the propagating phase. It can achieve independent phase control on two orthogonal circular polarizations and image at the desired angle. As a proof-of-concept, two broadband plasmonic metasurfaces are designed for multichannel holographic imaging in the near-infrared band (800–1200 nm). When two orthogonal circularly polarized waves are normally incident, the first plasmonic metasurface displays two different holograms in the vertical direction, thus achieving dual-channel imaging, and the second plasmonic metasurface can make two holograms deflected to the desired angle. It means that these two holograms can be displayed without overlapping for a linearly polarized wave, and the orientable three-channel holographic imaging is realized. Our work provides a general and compact scheme for multichannel orientable holographic imaging.

Predicting the electronic and mechanical properties of 2D diamond-like carbon and cubic boron nitride intercalated structures

Two-dimensional (2D) diamond-like (Diam) carbon (C) and cubic boron nitride (cBN) with different interface atoms named as BCB, BCN and NCN are investigated by first principal studies. The calculations reveal that the CB bond may be the most energetically favored bonding configuration in the Diam/cBN epitaxy. The electronic properties can be tuned not only by different interfacial atoms, but also by varying the number of intermediate C-layers. The application of negative stress transforms the 2D Diam/cBN intercalated structures from a semiconductor to a metal. The Young's and shear module indicate prominent mechanical properties for these intercalated structures, in particular for BCN that can be compared to bulk. The predicted 2D Diam/cBN intercalated structures presented here show that the stacking order, the number of intermediate C-layers, and the use of strain-induced structures can open new avenues for the application of novel ultra-thin and ultra-tough electronic reserve material devices.

The Tritube: Facilitating Upper Airway Surgery With an Ultrathin Cuffed Airway Device.

The Tritube is a narrow-bore cuffed tracheal tube (outer diameter 4.4 mm and inner diameter ~2.4 mm) that permits effective alveolar gas exchange using flow-controlled ventilation. Constant gas flow delivers physiological minute volumes, within preset pressure limits, and applies suction to the airway during expiration. The technique has attracted interest for laryngotracheal microsurgery as it provides superior surgical exposure and avoids many of the complications associated with high-frequency jet ventilation. Cuff inflation protects the lower airway and produces a motionless operating field. We describe the structure of the device, discuss its benefits, and suggest how it should be used clinically.

Greatly Enhanced Resonant Exciton-Trion Conversion in Electrically Modulated Atomically Thin WS2 at Room Temperature.

Excitonic resonance in atomically thin semiconductors offers a favorite platform to study 2D nanophotonics in both classical and quantum regimes and promises potentials for highly tunable and ultra-compact optical devices. The understanding of charge density dependent exciton-trion conversion is the key for revealing the underlaying physics of optical tunability. Nevertheless, the insufficient and inefficient light-matter interactions hinder the observation of trionic phenomenon and the development of excitonic devices for dynamic power-efficient electro-optical applications. Here, by engaging an optical cavity with atomically thin transition metal dichalcogenides (TMDCs), greatly enhanced exciton-trion conversion is demonstrated at room temperature (RT) and achieve electrical modulation of reflectivity of ≈40% at exciton and 7% at trion state, which correspondingly enables a broadband large phase tuning in monolayer tungsten disulfide. Besides the absorptive conversion, ≈100% photoluminescence conversion from excitons to trions is observed at RT, illustrating a clear physical mechanism of an efficient exciton-trion conversion for extraordinary optical performance. The results indicate that both excitons and trions can play significant roles in electrical modulation of the optical parameters of TMDCs at RT. The work shows the real possibility for realizing electrical tunable and multi-functional ultra-thin optical devices using 2D materials.

Recent advances in ultrathin materials and their applications in e‐skin

AbstractIntelligent technologies based on artificial intelligence and big data hold great potential for health monitoring and human–machine capability enhancement. However, electronics must be connected to the human body to realize this vision. Thus, tissue or skin‐like electronics with high stretchability and low stiffness mechanical properties are highly desirable. Ultrathin materials have attracted significant attention from the research community and the industry because of their high performance and flexibility. Over the past few years, considerable progress has been made in flexible ultrathin sensors and devices based on ultrathin materials. Here, we review the developments in this area and examine representative research progress in ultrathin materials fabrication and device construction. Strategies for the fabrication of stretchable ultrathin materials and devices are considered. The relationship between the thin‐film structure and performance is emphasized and highlighted. Finally, the current capabilities and limitations of ultrathin devices were explored.image

Janus monolayer ScXY (X≠Y = Cl, Br and I) for piezoelectric and valleytronic application: a first-principle prediction

Coexistence of ferromagnetism, piezoelectricity and valley in two-dimensional (2D) materials is crucial to advance multifunctional electronic technologies. Here, Janus ScXY (X≠Y = Cl, Br and I) monolayers are predicted to be piezoelectric ferromagnetic semiconductors with dynamical, mechanical and thermal stabilities. They all show an in-plane easy axis of magnetization by calculating magnetic anisotropy energy (MAE) including magnetocrystalline anisotropy energy and magnetic shape anisotropy energy. The MAE results show that they intrinsically have no spontaneous valley polarization. The predicted piezoelectric strain coefficients d 11 and d 31 (absolute values) are higher than ones of most 2D materials. Moreover, the d 31 (absolute value) of ScClI reaches up to 1.14 pm V−1, which is highly desirable for ultrathin piezoelectric device application. To obtain spontaneous valley polarization, charge doping are explored to tune the direction of magnetization of ScXY. By appropriate hole doping, their easy magnetization axis can change from in-plane to out-of-plane, resulting in spontaneous valley polarization. Taking ScBrI with 0.20 holes per f.u. as an example, under the action of an in-plane electric field, the hole carriers of K valley turn towards one edge of the sample, which will produce anomalous valley Hall effect, and the hole carriers of Γ valley move in a straight line. These findings could pave the way for designing piezoelectric and valleytronic devices.

Significant enhancement in thermoelectric performance of S-shaped germanene nanoribbon devices

Due to the advantages including high efficiency and environmental friendliness, the two-dimensional thermoelectric materials have received huge attention. However, the energy conversion efficiency is not high enough for application. In this work, we have theoretically investigated the thermoelectric performance for both zigzag (ZGeNRs) and armchair (AGeNRs) germanene nanoribbons with edge defects by nonequilibrium Green's function method. For perfect germanene nanoribbons (GeNRs), it is shown that with its width increasing, the maximum of ZT values (ZTmax) decreases monotonously while the phononic thermal conductance increases linearly. For both ZGeNRs and AGeNRs, with increasing defect numbers in longitudinal direction, ZTmax increases monotonously while the phononic thermal conductance decreases. In particular, by introducing periodic defects, the ZTmax of both ZGeNRs and AGeNRs has been raised to above 2, which is due to higher Seebeck coefficient and lower thermal conductance. Our findings provide an effective way to enhance thermoelectric performance, which is beneficial for the design of ultra-thin thermoelectric devices.

Enhanced capillary performance of hierarchical dendritic copper surface for ultrathin heat-spreading devices

High-performance electronic systems require efficient heat dissipation devices. Vapor chambers (VCs) are practical thermal solutions for lightweight portable electronics that have limited heat dissipation space. A functional VC requires an interior wick to sustain the capillary circulation of the condensed fluid back to the heated region. The capillary wick performance can be indexed by the ratio of liquid permeability to the effective pore size of the wick structure. This study describes the capillary behavior of a thin hierarchical dendritic copper film (<100 μm thick) prepared by electrodeposition and thermal sintering. The effects of the electrodeposition current density, deposition time, and sintering temperature on the capillary performance of the dendritic copper films were investigated. The relationship between the wicking capability and dendritic morphology, tailored by the electrodeposition process, was explored. A post-deposition sintering treatment was found to be beneficial for improving the structural integrity, adhesion, and capillary performance of dendritic copper wicks. A very high capillary performance (0.81 μm) was realized on a 30-μm-thick dendritic copper wick that matches the need for ultrathin VCs used in highly compact electronic systems.

Comprehensive mechanical performance characteristics of newly developed ultra-thin coronary stents

Endovascular technologies such as percutaneous transluminal coronary angioplasty and stenting are widely used due to being minimally invasive procedures. One of the endovascular devices, a drug-eluting stent (DES), proved to be a safe and effective therapeutic option for cardiologists, substantially reducing in-stent restenosis (ISR) rates. DES consists of thin (< 200 µm) or ultra-thin (< 70 µm) struts made of Cobalt-Chromium (CoCr) or other alloys. The DES with ultra-thin struts exhibits higher longitudinal flexibility and improved trackability. It also reduces vascular injury and promotes more rapid endothelization, substantially decreasing the risk of ISR. This study evaluates comprehensive mechanical performances of two different structural designs of newly developed ultra-thin CoCr stents (i.e., strut width of 75 µm or 65 µm). These two stent designs are engineered with distinct bridge size and distribution patterns, prioritizing enhanced flexibility irrespective of the strut thickness. Computational modeling results show anticipated stress distribution with maximum local stresses, which are compared with experimental outcomes conducted in this study. ASTM guideline based in vitro experimental studies have been conducted to assess the stents’ longitudinal flexibility, radial strength, recoil and shortening, as well as crushability through this study. Three-point bending experiments have demonstrated that the stent design 1 requires less bending force compared to the design 2, showing better longitudinal flexibility of the stent design 1 in the collapsed state for the new ultra-thin DES devices. The design 1 also showed superior radial strength and recoiling phenomena, as well as smaller shortening. In short, the stent design 1 shows better mechanical performance needed for the coronary artery DES.

Metal‐Organic Framework Thin Films Grown on Functionalized Graphene as Solid‐State Ion‐Gated FETs

AbstractThe unique properties of 2D‐materials like graphene are exploited in various electronic devices. In sensor applications, graphene shows a very high sensitivity, but only a low specificity. This shortcoming can be mastered by using heterostructures, where graphene is combined with materials exhibiting high analyte selectivities. Herein, this study demonstrates the precise deposition of nanoporous metal‐organic frameworks (MOFs) on graphene, yielding bilayers with excellent specificity while the sensitivity remains large. The key for the successful layer‐by‐layer deposition of the MOF films (SURMOFs) is the use of planar polyaromatic anchors. Then, the MOF pores are loaded with ionic liquid (IL). For functioning sensor devices, the IL@MOF films are grown on graphene field‐effect transistors (GFETs). Adding a top‐gate electrode yields an ion‐gated GFET. Analysis of the transistor characteristics reveals a clear Dirac point at low gate voltages, good on‐off ratios, and decent charge mobilities and densities in the graphene channel. The GFET‐sensor reveals a strong and selective response. Compared to other ion‐gated‐FET devices, the IL@MOF material is relatively hard, allowing the manufacturing of ultrathin devices. The new MOF‐anchoring strategy offers a novel approach generally applicable for the functionalization of 2D‐materials, where MOF/2D‐material hetero‐bilayers carry a huge potential for a wide variety of applications.

Two-dimensional MoS2/Ti2CX2 (X = S, Se, Te) heterostructures with tunable electrical contact type

The tunable electrical contact type in van der Waals (vdW) heterostructures plays an important role in promoting the efficiency of ultrathin electronic devices. From the first-principles calculations, the electronic properties and electrical contact type of the MoS2/Ti2C heterostructure and its ramifications MoS2/Ti2CX2 (X = S, Se, Te) heterostructures are verified. The strong interfacial interaction is found in pristine MoS2/Ti2C heterostructure, resulting in the metallization of MoS2 layer. The functionalization of S, Se, and Te atoms can weaken the interfacial interaction, forming metal-semiconductor contact. The p-type Schottky contact is formed in MoS2/Ti2CS2 and MoS2/Ti2CSe2 heterostructures, while Ohmic contact is formed in MoS2/Ti2CTe2 heterostructure. The application of external electric field can efficiently modulate the contact type of MoS2/Ti2CX2 (X = Se and Te) heterostructures. The discrepancy of electron affinity of S, Se, and Te atoms leads to the different interlayer charge transfer, and thus the different electrical contact type can be realized in MoS2/Ti2CX2 (X = S, Se, Te) heterostructures. The tunable contact type makes these heterostructures become potential candidate materials for enhancing the nanoelectronic devices’ efficiency.

Extreme Air Sensitivity and Nonself-Limited Oxidation of Two-Dimensional Magnetic Tellurides

The two-dimensional (2D) self-intercalated van der Waals magnets, chromium tellurides (Cr1+δTe2), with room temperature ferromagnetism and exotic topological spin textures, have emerged as an attractive platform for the development of ultrathin spintronic devices. While many prior studies claim Cr1+δTe2 are air-stable, which is crucial for practical applications, we demonstrate that within only minutes of exposure to air dramatic changes occur in the Raman and X-ray photoelectron spectroscopy spectra of nanoplates at room temperature. Time-dependent magnetization measurements and transmission electron microscopy studies suggested the rapid oxidation is not self-limited and has distinct processes on the surfaces and edges. Density functional theory calculations confirmed the spontaneous oxygen adsorption on the surface and the thermodynamically favorable oxidation process. These findings demonstrate unambiguously that the surface of Cr1+δTe2 is extremely air-sensitive, highlighting the necessity of surface protection for fundamental studies of intrinsic 2D magnetism and practical applications of advanced spintronic devices.

Reproduced out-of-plane ferroelectricity in monolayer SnTe van der Waals heterostructures

Due to the shrinking in size of nonvolatile memory devices, the two-dimensional ferroelectric van der Waals (vdW) heterostructures have received huge attention. However, it is still difficult to maintain the out-of-plane (OOP) ferroelectricity. In this work, we have theoretically investigated the relationship between the ferroelectricity and the strain of bulk and few-layer SnTe by first-principles calculations. The results indicate that the α-SnTe can exist stably within the strain between −6% and 6%, and the complete OOP polarization occurs within the strain between −4% and −2%. Unfortunately, the OOP polarization disappears while the bulk α-SnTe is thinned to a few layers. However, the complete OOP polarization recurs in monolayer SnTe/PbSe vdW heterostructures, which is due to the strong interface coupling. Our findings provide an effective way to enhance ferroelectric performance, which is beneficial for the design of ultra-thin ferroelectric devices.

Surface magneto-optics in yttrium iron garnets

We report a study of surface reconstruction effects in yttrium iron garnets using density functional theory. This work responds to the need to explain the physical basis of recent experimental results showing a significant enhancement in Faraday rotation in iron garnets. These materials are extensively used in the telecom industry for nonreciprocal device applications. Understanding the physical basis of the heightened Faraday response at the surface is important for the development of ultrathin nonreciprocal devices. Our results show that the bandgap near the surface is significantly reduced compared to that of the bulk, and that spin-orbit coupling effects become more important near the surface. We find that the decrease in band gap results in an enhancement and change in direction in the Faraday rotation in the visible range all the way to the deep red. Electronic transition matrix elements for surface-sensitive ultra-thin layers are calculated and compared to those of bulk samples, leading to the conclusion that the octahedrally-oxygen-coordinated iron sublattice in these ferrimagnetic materials is more strongly affected by surface reconstruction than the other, anti-ferromagnetically coupled, tetrahedral sublattice. We explain how these changes contribute to the enhancement in magneto-optic response near the surface as compared to the bulk.