N-type Monolayer MoS2 Research Articles

Tuning the electronic structure of monolayer MoS2 towards metal like via vanadium doping

Doping of two-dimensional layered semiconducting materials is becoming pivotal in tailoring their electronic properties, enabling the development of advanced electronic and optoelectronic devices, where the selection of dopant is important. Here, we demonstrate the potential of substitutional vanadium (V) doping in monolayer molybdenum disulfide (MoS2) lattice in different extents leading to tunable electronic and optoelectronic properties. We found that low-level V doping (∼1 at.%) induces p-type characteristics in otherwise n-type monolayer MoS2, whereas medium-level doping (∼5 at.%) leads to an ambipolar semiconductor. Degenerately doped MoS2 (∼9 at.%) facilitates a transition from semiconducting towards metallic (metal-like) with reduced electrical resistivity (∼4.5 Ωm of MoS2 to ∼2.2×10−5Ωm), low activation energy for transport (∼11 meV), and electric field independent drain current in field effect transistor–based transfer characteristics. A detailed temperature- and power-dependent photoluminescence study along with density functional theory–based calculations in support unravels the emergence of an excitonic transition at ∼850 nm with its intensity dependent on the amount of vanadium. This study shows the potential of V doping in MoS2 for generating multifunctional two-dimensional materials for next generation electronics, optoelectronics, and interconnects with systematic control over its electronic structure in a wide range. Published by the American Physical Society 2024

Monolithic 3D integration of back-end compatible 2D material FET on Si FinFET

The performance enhancement of integrated circuits relying on dimension scaling (i.e., following Moore’s Law) is more and more challenging owing to the physical limit of Si materials. Monolithic three-dimensional (M3D) integration has been considered as a powerful scheme to further boost up the system performance. Two-dimensional (2D) materials such as MoS2 are potential building blocks for constructing upper-tier transistors owing to their high mobility, atomic thickness, and back-end-of-line (BEOL) compatible processes. The concept to integrate 2D material-based devices with Si field-effect transistor (FET) is technologically important but the compatibility is yet to be experimentally demonstrated. Here, we successfully integrated an n-type monolayer MoS2 FET on a p-type Si fin-shaped FET with 20 nm fin width via an M3D integration technique to form a complementary inverter. The integration was enabled by deliberately adopting industrially matured techniques, such as chemical mechanical planarization and e-beam evaporation, to ensure its compatibility with the existing 3D integrated circuit process and the semiconductor industry in general. The 2D FET is fabricated using low-temperature sequential processes to avoid the degradation of lower-tier Si devices. The MoS2 n-FETs and Si p-FinFETs display symmetrical transfer characteristics and the resulting 3D complementary metal-oxide-semiconductor inverter show a voltage transfer characteristic with a maximum gain of ~38. This work clearly proves the integration compatibility of 2D materials with Si-based devices, encouraging the further development of monolithic 3D integrated circuits.

Metal contacts with Moire interfaces on WSe2 for ambipolar applications

The rational design of metal contacts on transition metal dichalcogenides can significantly improve the performance of 2D devices. We have previously shown that a Moire interface between n-type monolayer MoS2 and metal contacts enhances the stability of physisorptive interface sites, thereby enabling weaker Fermi level pinning and allowing easier variation of the Schottky barrier height at these interfaces. We extend these calculations to p-type and ambipolar WSe2 contacts in this work. The analysis shows that the Moire interfaces again have a weaker Fermi level pinning, while most metals have chemisorptive sites with stronger pinning. We find that the most stable site of Pd is a Moire site with an unusually low p-type Schottky barrier height (p-SBH), while Au has a metastable low p-SBH. In and Al retain their low n-type SBHs, which together with Pd enable ambipolar contacts by the choice of contact metals, indicating that WSe2 can be used for high-performance ambipolar devices with the rational design of contact metals.

Tunable friction of monolayer MoS2 by control of interfacial chemistry

In this report, we utilize the heterostructures of monolayer MoS2 and self-assembled monolayers (SAMs) of organic molecules as a platform to understand how carrier density of nanomaterials affects their friction behaviors. Previous studies on the friction of two-dimensional materials have explored the effect of lattice structures and morphologies. Given the same normal force and scanning speed of the AFM tip, we observe the sliding friction of high-quality n-type monolayer MoS2 could be reduced by SAMs. The friction tunability is attributed to the charge transfer from MoS2to SAMs, which modulated the carrier density and hence the carrier-mediated friction behaviors in 2D MoS2. ab initio simulations and nanoscale mapping of work functions support this hypothesis. These SAMs-based heterostructures provide a potential tool to control friction in low-dimensional materials, and also an original perspective on the effect of electron–phonon coupling on friction at nanoscale.

Effects of uniaxial tensile strain on the electron–phonon scattering limited carrier mobility in an n-type monolayer MoS2 at room temperature: first-principles calculations

In the present work, we theoretically study the strain effect on the room temperature mobility (RTM) of a n-type Monolayer MoS2 limited by electron–phonon (e–ph) scattering. Our numerical results indicate that the RTM along zigzag direction of such a 2D material can be efficiently modulated by a uniaxial tensile strain. Such an RTM, denoted as , has a sizable reduction (enhancement) as a moderate tensile strain is applied in a parallel (perpendicular) direction. For example, when the strain strength amounts to 7%, the two distinct cases of the strain applied in x and y directions differ from each other by roughly two times. In contrast, the RTM in armchair direction is not so sensitive to a tensile strain. The underlying mechanism for such a strain effect on the mobility is then analysed in depth. Our results are obtained completely on the level of first-principles calculations, free from any empirical simplifications. Therefore, our above findings provide reliable and detailed information for experimentally manipulating the RTM of a n-type monolayer MoS2 by simply stretching the sample.

Complementary inverters based on low-dimensional semiconductors prepared by facile and fully scalable methods

Two-dimensional (2D) semiconductors offer great potential in nanoelectronics due to their unique electrical and optical properties mainly attributed to their atomically thin nature. To make use of 2D semiconductors for practical applications, it is highly desired to develop scalable methods for integration of complementary circuits rather than a discrete device. We report a simple, scalable method for fabricating complementary inverters based on n-type monolayer MoS2, grown by a chemical vapor deposition (CVD), and inkjet-printed p-type SWCNTs. The CVD and inkjet printing methods are effectively combined to show the feasibility in terms of the integration of low-dimensional materials for complementary circuits. In the complementary circuits, cost-effectively printed Ag is utilized as the source and drain electrodes for both MoS2 and SWCNT transistors. Both the n- and p-type transistors show decent device characteristics at low operating voltages under ambient conditions. As a result, the complementary inverter composed of MoS2 and SWCNT transistors exhibits excellent performance with a gain over 10, low power dissipation, high noise margins, and switching threshold of 1/2 VDD at a low operating voltage. This successful demonstration of the complementary inverter, the most basic building block in digital circuits, will lead promising 2D semiconductors to practical applications.

Effects of high-k dielectric environment on the full ballistic transport properties of monolayer MoS2 FETs

High-k dielectric materials are indispensable as gate layers for micro- and nano-electronic devices. Using first-principles calculations and non-equilibrium Green's function simulations, we studied the electrical transport characteristics of p-type and n-type monolayer MoS2 field effect transistors (FETs) under various gate dielectric environments. We found that the intrinsic dielectric property of the gate insulator played an important role in the transport performance of nanodevices. For both types of MoS2 FETs, a high-k gate insulator enhances the current on/off ratio and reduces the subthreshold swing by properly shifting the valence (p-type) or conduction (n-type) bands around the bias energy window, which has benefits for the design of MoS2-based short-channel nanodevices in the future.

Ultrafast Exciton Dissociation and Long-Lived Charge Separation in a Photovoltaic Pentacene–MoS2 van der Waals Heterojunction

van der Waals heterojunctions between two-dimensional (2D) layered materials and nanomaterials of different dimensions present unique opportunities for gate-tunable optoelectronic devices. Mixed-dimensional p-n heterojunction diodes, such as p-type pentacene (0D) and n-type monolayer MoS2 (2D), are especially interesting for photovoltaic applications where the absorption cross-section and charge transfer processes can be tailored by rational selection from the vast library of organic molecules and 2D materials. Here, we study the kinetics of excited carriers in pentacene-MoS2 p-n type-II heterojunctions by transient absorption spectroscopy. These measurements show that the dissociation of MoS2 excitons occurs by hole transfer to pentacene on the time scale of 6.7 ps. In addition, the charge-separated state lives for 5.1 ns, up to an order of magnitude longer than the recombination lifetimes from previously reported 2D material heterojunctions. By studying the fractional amplitudes of the MoS2 decay processes, the hole transfer yield from MoS2 to pentacene is found to be ∼50%, with the remaining holes undergoing trapping due to surface defects. Overall, the ultrafast charge transfer and long-lived charge-separated state in pentacene-MoS2 p-n heterojunctions suggest significant promise for mixed-dimensional van der Waals heterostructures in photovoltaics, photodetectors, and related optoelectronic technologies.

The electrical and valley properties of monolayer MoSe2

We studied the electrical, optical and valley properties of monolayer MoSe2. The measured PL circular polarization of monolayer MoSe2 was negligibly small, compared with the more-than 60% circular polarization of monolayer MoS2. Doping-dependent PL measurement and Kelvin probe microscopy show that the small circular polarization because of its longer exciton lifetime in monolayer MoSe2. Furthermore, it is found that the longer exciton lifetime is due to the nearly intrinsic electrical property of monolayer MoSe2, where non-radiative trion decay, which is the dominant exciton decay mechanism in n-type monolayer MoS2, is minimized due to the low density of excess electrons (holes). We believe that S and Se vacancies play an important role in determining the Fermi level position for these materials. Our experimental results show that monolayer MoSe2 can be a better valleytronics material than monolayer MoS2 in spite of its small circular polarization.

3D Band Diagram and Photoexcitation of 2D-3D Semiconductor Heterojunctions.

The emergence of a rich variety of two-dimensional (2D) layered semiconductor materials has enabled the creation of atomically thin heterojunction devices. Junctions between atomically thin 2D layers and 3D bulk semiconductors can lead to junctions that are fundamentally electronically different from the covalently bonded conventional semiconductor junctions. Here we propose a new 3D band diagram for the heterojunction formed between n-type monolayer MoS2 and p-type Si, in which the conduction and valence band-edges of the MoS2 monolayer are drawn for both stacked and in-plane directions. This new band diagram helps visualize the flow of charge carriers inside the device in a 3D manner. Our detailed wavelength-dependent photocurrent measurements fully support the diagrams and unambiguously show that the band alignment is type I for this 2D-3D heterojunction. Photogenerated electron-hole pairs in the atomically thin monolayer are separated and driven by an external bias and control the "on/off" states of the junction photodetector device. Two photoresponse regimes with fast and slow relaxation are also revealed in time-resolved photocurrent measurements, suggesting the important role played by charge trap states.

Acoustic phonon assisted free-carrier optical absorption in an n-type monolayer MoS2 and other transition-metal dichalcogenides

The theory of free-carrier absorption (FCA) is given for monolayers of transition-metal dichalcogenides, particularly for molybdenum disulphide (MoS2), when carriers are scattered by phonons. Explicit expressions for the absorption coefficient α are obtained and discussed for acoustic phonon scattering via screened deformation potential and piezoelectric coupling taking polarization of the radiation in the plane of the layer. It is found that α monotonously decreases with the increasing photon frequency Ω, increases with the increasing temperature T, and linearly depends on two-dimensional electron concentration ns. Effect of screening, which is ignored in all the earlier FCA studies, is found to reduce α significantly, attributing to the larger effective mass of the electrons. Results are also obtained in the classical and quantum limit giving the power laws α ∼ Ω−2 and T. Comparison of the results is made with those in bulk semiconductors and semiconductor quantum wells.

Magnetic control of valley and spin degrees of freedom via magnetotransport in n -type monolayer MoS2

We study the quantum magnetotransport properties at low temperature for n-type monolayer MoS2 under a perpendicular magnetic field within the linear response theory. By using Kubo formula, it is predicted that the valley- and spin-resolved Hall conductance obeys the same rule as that for conventional fermions. However, the first Hall plateau and the longitudinal charge conductance in low energies are fully valley/spin-polarized, which may produce a pure valley/spin current. In the low magnetic field limit, importantly, the resistance as a function of field is in good agreement with the available experiment data. These interesting findings can be tested experimentally and may be useful for valleytronic and spintronic applications.

Terahertz Optical Conductance of n-type Monolayer MoS2

We present a theoretical study on optoelectronic properties of n-type monolayer MoS2 under long-wavelength light radiation. The optical conductance for such material system is evaluated using the energy-balance equation derived from the Boltzmann equation in the presence of linearly polarized radiation field. For n-type monolayer MoS2, the optical absorption in terahertz bandwidth can be achieved via intra spin-conserved transitions and inter spin-flip transitions at K and K′ valleys. More importantly, we find that there is a terahertz absorption peak in the radiation frequency range 2–10 THz. The intensity and width of this absorption peak depend sensitively on temperature and the position of the peak can be tuned via changing the electron density. This study shows that n-type monolayer MoS2 can be applied as frequency tunable THz optical and optoelectrical devices.

Light generation and harvesting in a van der Waals heterostructure.

Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our p–n diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.

Fully valley- and spin-polarized magnetocapacitance in n-type monolayer MoS2

We present a theoretical investigation on the quantum magnetocapacitance (MC) for n-type monolayer MoS2 under a perpendicular magnetic field. We find that the MC clearly reflects the valley- and spin-resolved Landau levels (LLs). Interestingly, the MC is fully valley- and spin-polarized, which results in perfect square-wave-shaped polarization. This fully valley- and spin-polarized MC, especially the peak corresponding to the lowest LL, may be of great significance in valleytronic and spintronic device applications because it provides a magnetic method to control the electron valley and spin degrees of freedom. The MC behavior as a function of the magnetic field is also discussed.