Semiconductors require stable doping for applications in transistors, optoelectronics, and thermoelectrics. However, this has been challenging for two-dimensional (2D) materials, where existing approaches are either incompatible with conventional semiconductor processing or introduce time-dependent, hysteretic behavior. Here we show that low-temperature (<200 °C) substoichiometric AlOx provides a stable n-doping layer for monolayer MoS2, compatible with circuit integration. This approach achieves carrier densities >2 × 1013 cm-2, sheet resistance as low as ∼7 kΩ/□, and good contact resistance ∼480 Ω·μm in transistors from monolayer MoS2 grown by chemical vapor deposition. We also reach record current density of nearly 700 μA/μm (>110 MA/cm2) along this three-atom-thick semiconductor while preserving transistor on/off current ratio >106. The maximum current is ultimately limited by self-heating (SH) and could exceed 1 mA/μm with better device heat sinking. With their 0.1 nA/μm off-current, such doped MoS2 devices approach several low-power transistor metrics required by the international technology roadmap.

Layering two-dimensional van der Waals materials provides a high degree of control over atomic placement, which could enable tailoring of vibrational spectra and heat flow at the sub-nanometer scale. Here, using spatially resolved ultrafast thermoreflectance and spectroscopy, we uncover the design rules governing cross-plane heat transport in superlattices assembled from monolayers of graphene (G) and MoS2 (M). Using a combinatorial experimental approach, we probe nine different stacking sequences, G, GG, MG, GGG, GMG, GGMG, GMGG, GMMG, and GMGMG, and identify the effects of vibrational mismatch, interlayer adhesion, and junction asymmetry on thermal transport. Pure G sequences display evidence of quasi-ballistic transport, whereas adding even a single M layer strongly disrupts heat conduction. The experimental data are described well by molecular dynamics simulations, which include thermal expansion, accounting for the effect of finite temperature on the interlayer spacing. The simulations show that an increase of ∼2.4% in the layer separation of GMGMG, relative to its value at 300 K, can lead to a doubling of the thermal resistance. Using these design rules, we experimentally demonstrate a five-layer GMGMG superlattice "thermal metamaterial" with an ultralow effective cross-plane thermal conductivity comparable to that of air.

Electrical contact resistance to two-dimensional (2D) semiconductors such as monolayer MoS_2 is a key bottleneck in scaling the 2D field effect transistors (FETs). The 2D semiconductor in contact with three-dimensional metal creates unique current crowding that leads to increased contact resistance. We developed a model to separate the contribution of the current crowding from the intrinsic contact resistivity. We show that current crowding can be alleviated by doping and contact patterning. Using Landauer-Büttiker formalism, we show that van der Waals (vdW) gap at the interface will ultimately limit the electrical contact resistance. We compare our models with experimental data for doped and undoped MoS_2 FETs. Even with heavy charge-transfer doping of > 2x10^13 cm^-2, we show that the state-of-the-art contact resistance is 100 times larger than the ballistic limit. Our study highlights the need to develop efficient interface to achieve contact resistance of < 10 Ω.μm, which will be ideal for extremely scaled devices.

The future scaling of semiconductor devices can be continued only by the development of novel nanofabrication techniques and atomically thin transistor channels. Here we demonstrate ultra-scaled MoS2 field-effect transistors (FETs) realized by a shadow evaporation method which does not require nanofabrication. The method enables large-scale fabrication of MoS2 FETs with fully gated  ∼10 nm long channels. The realized ultra-scaled MoS2 FETs exhibit very small hysteresis of current–voltage characteristics, high drain currents up to  ∼560 A m−1, very good drain current saturation for such ultra-short devices, subthreshold swing of ∼120 mV dec−1, and drain current on/off ratio of ∼106 in air ambient. The fabricated ultra-scaled MoS2 FETs are also used to realize logic gates in n-type depletion-load technology. The inverters exhibit a voltage gain of  ∼50 at a power supply voltage of only 1.5 V and are capable of in/out signal matching.

The temperature coefficient of resistance (TCR) of thin metal lines is often used for applications in thermometry, bolometers, or thermal accelerometers. However, metal TCR is much degraded in nanometer-thin films due to strong surface scattering, preventing their use as fast thermal sensors, which simultaneously require low thermal mass and large TCR. In contrast, here we show that the TCR of doped two-dimensional (2D) semiconductors is large (∼0.3% K−1 at 300 K in MoS2 and MoTe2) even at sub-nanometer thickness. This is larger than that of any metals with thicknesses up to ∼35 nm and larger than that of ∼95 nm thick Cu lines (0.25% K−1) at 300 K. At 100 K, the TCR of these 2D materials is doubled, ∼0.6% K−1. Comparison with detailed 2D transport models suggests that the TCR could be further enhanced (up to 0.45% K−1 at 300 K and ∼2.5% K−1 at 100 K) by reducing the density of Coulomb impurities and scattering centers. Such high TCR in atomically thin 2D semiconductors could lead to the design of fast thermal sensors.

The temperature coefficient of resistance (TCR) of thin metal lines is often used for applications in thermometry [1], phase change [2], or even thermal accelerometers [3]. However, the TCR of metals drops sharply in films thinner than ~10 nm due to strong surface scattering, preventing their use as ultra-fast thermal sensors, which require reduced thermal mass. In contrast, here we demonstrate for the first time that atomically-thin two-dimensional (2D) materials maintain large TCRs ( >0.5%) even at sub-nm thickness. Fundamentally, these experiments illustrate that transport in high-quality 2D materials remains phonon-limited, unlike ultra-thin metals. From an application standpoint, high TCR in atomically-thin films is expected to enable fast thermal sensors.

Ternary content-addressable memory (TCAM) is specialized hardware that can perform in-memory search and pattern matching for data-intensive applications. However, achieving TCAMs with high search capacity, good area efficiency and good energy efficiency remains a challenge. Here, we show that two-transistor–two-resistor (2T2R) transition metal dichalcogenide TCAM (TMD-TCAM) cells can be created by integrating single-layer MoS 2 transistors with metal-oxide resistive random-access memories (RRAMs). The MoS 2 transistors have very low leakage currents and can program the RRAMs with exceptionally robust current control, enabling the parallel search of very large numbers of data bits. These TCAM cells also exhibit remarkably large resistance ratios ( R -ratios) of up to 8.5 × 10 5 between match and mismatch states. This R -ratio is comparable to that of commercial TCAMs using static random-access memories (SRAMs), with the key advantage that our 2T2R TCAMs use far fewer transistors and have zero standby power due to the non-volatility of RRAMs.

Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS 2 , and WSe 2 with thermal resistance greater than 100 times thicker SiO 2 and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.

We investigate four monolayer transition metal dichalcogenides (TMDs) - MoS 2 , WS 2 , MoSe 2 , and WSe 2 - transferred to silicon substrates as possible sub-nanometer copper diffusion barriers compatible with back-end-of-line temperatures. Based on top-down scanning electron microscope (SEM) and cross-section transmission electron microscope imaging, we demonstrate that the W-based TMDs act as diffusion barriers up to 360°C, while Mo-based TMDs fail at temperatures as low as 300°C. Analysis by SEM indicates that points of failure occur as pinholes, suggesting that mechanical damage may be the origin of failure. Further analysis by X-ray photoemission spectroscopy on as-grown TMDs reveals all four to be chemically unaffected by copper at temperatures as high as 600°C, indicating that directly-grown TMDs still have potential as sub-nanometer diffusion barriers.

Mos 2 is a two-dimensional (2D) semi-conductor which is now considered as a channel material in 2D FETs [1]-[3]. Recently we found that single-layer (1L) Mos 2 FETs grown by chemical vapour deposition (CVD) and encapsulated with high-quality Al 2 O 3 layer exhibit superior performance and reliability compared to all previously reported 2D FETs [2]. However, while the I on / I off ratio (~10 9 ) of the devices [2] already fulfills commercial standards, their reliability is still poorer than for Si devices [4]. In order to understand the physical origin of these aspects, here we analyze the impact of annealing at 300°C and Al 2 O 3 deposition on the performance and hysteresis dynamics of 1L MoS 2 FETs.

Methylation on DNA is an epigenetic modification of DNA in which methyl groups are added at the 5-carbon position of cytosine. Aberrant DNA methylation, which has been associated with carcinogenesis, can be assessed in various human biological fluids and potentially be used as biomarkers for detection of cancer at early-stage. Analytically sensitive and specific assays for methylation targeting low-abundance and fragmented DNA are needed for optimal clinical diagnosis and prognosis. We present a solid-state nanopore-based direct methylation detection assay that circumvents bisulfite conversion and PCR amplification. We used methyl-binding proteins (MBPs), which selectively label the methylated DNA. The nanopore-based assay selectively detects methylated DNA/MBP complexes through a 19 nm nanopore with significantly deeper and prolonged nanopore ionic current blocking, while unmethylated DNA molecules were not detectable due to their smaller diameter. Discrimination of hypermethylated and unmethylated DNA on 90 bp, 60 bp, and 30 bp DNA fragments was demonstrated using sub 10 nm nanopores. Hypermethylated DNA fragments fully bound with MBP are differentiated from unmethylated DNA at 2.1-fold to 6.5-fold current blockades and 4.5-fold to 23.3-fold transport durations. These nanopore assays can also detect CpG dyads in DNA fragments and could someday profile the position of methylated CpG sites on DNA fragments. Furthermore, we present a novel nanopore-based assay using a nanopore in a MoS2 membrane. We show that the dsDNA translocation was effectively slowed down using an asymmetric concentration of buffer and explore the possibility of profiling the position of methylcytosines on the DNA strands as they translocate through the 2D membrane. Our findings advance us one step closer towards the possible use of nanopore sensing technology in medical applications such as cancer detection.

Understanding growth, grain boundaries (GBs), and defects of emerging two-dimensional (2D) materials is key to enabling their future applications. For quick, nondestructive metrology, many studies rely on confocal Raman spectroscopy, the spatial resolution of which is constrained by the diffraction limit (similar to 0.5 mu m). Here we use tip-enhanced Raman spectroscopy (TERS) for the first time on synthetic MoSe2 monolayers, combining it with other scanning probe microscopy (SPM) techniques, all with sub-20 nm spatial resolution. We uncover strong nanoscale heterogeneities in the Raman spectra of MoSe2 transferred to gold substrates [one near 240 cm(-1) (A(1)'), and others near 287 cm(-1) (E'), 340 cm(-1), and 995 cm(-1)], which are not observable with common confocal techniques and appear to imply the presence of nanoscale domains of MoO3. We also observe strong tip-enhanced photoluminescence (TEPL), with a signal nearly an order of magnitude greater than the far-field PL. Combining TERS with other SPM techniques, we find that GBs can cut into larger domains of MoSe2, and that carrier densities are higher at GBs than away from them.

This invited talk will present recent highlights from our research on two-dimensional (2D) materials including graphene, boron nitride (h-BN), and transition metal dichalcogenides (TMDs). The results span from fundamental measurements and simulations, to device-and several unusual system-oriented applications which take advantage of unique 2D material properties. Basic electrical, thermal, and thermoelectric properties of 2D materials will also be discussed.

11:20 – 11:40 Mapping the Operational Landscape of microRNAs in Synthetic Gene Circuits Tyler Quarton, Leonidas Bleris Bioengineering, The University of Texas at Dallas MicroRNAs are a class of short, noncoding RNAs that are ubiquitous modulators of gene expression, with roles in development, homeostasis, and disease. Engineered microRNAs are now frequently used as regulatory modules in synthetic biology. Moreover, synthetic gene circuits equipped with engineered microRNA targets with perfect complementarity to endogenous microRNAs establish an interface with the endogenous milieu at the single-cell level. The function of engineered microRNAs and sensor systems is typically optimized through extensive trial-and-error. Here, using a combination of synthetic biology experimentation in human embryonic kidney cells and quantitative analysis, we investigate the relationship between input genetic template abundance, microRNA concentration, and output under microRNA control. We provide a framework that employs the complete operational landscape of a synthetic gene circuit and enables the stepwise development of mathematical models. We derive a phenomenological model that recapitulates experimentally observed nonlinearities and contains features that provide insight into the microRNA function at various abundances. Our work facilitates the characterization and engineering of multi-component genetic circuits and specifically points to new insights on the operation of microRNAs as mediators of endogenous information and regulators of gene expression in synthetic biology.

Methylation at the 5-carbon position of the cytosine nucleotide base in DNA has been shown to be a reliable diagnostic biomarker for carcinogenesis. Early detection of methylation and intervention could drastically increase the effectiveness of therapy and reduce the cancer mortality rate. Current methods for detecting methylation involve bisulfite genomic sequencing, which are cumbersome and demand a large sample size of bodily fluids to yield accurate results. Hence, more efficient and cost effective methods are desired. Based on our previous work, we present a novel nanopore-based assay using a nanopore in a MoS2 membrane, and the methyl-binding protein (MBP), MBD1x, to detect methylation on dsDNA. We show that the dsDNA translocation was effectively slowed down using an asymmetric concentration of buffer and explore the possibility of profiling the position of methylcytosines on the DNA strands as they translocate through the 2D membrane. Our findings advance us one step closer towards the possible use of nanopore sensing technology in medical applications such as cancer detection.

Van der Waals heterostructures consisting of two-dimensional materials offer a platform to obtain materials by design and are very attractive owing to unique electronic states. Research on 2D van der Waals heterostructures (vdWH) has so far been focused on fabricating individually stacked atomically thin unary or binary crystals. Such systems include graphene, hexagonal boron nitride, and members of the transition metal dichalcogenide family. Here we present our experimental study of the optoelectronic properties of a naturally occurring vdWH, known as franckeite, which is a complex layered crystal composed of lead, tin, antimony, iron, and sulfur. We present here that thin film franckeite (60 nm < d < 100 nm) behaves as a narrow band gap semiconductor demonstrating a wide-band photoresponse. We have observed the band-edge transition at ∼1500 nm (∼830 meV) and high external quantum efficiency (EQE ≈ 3%) at room temperature. Laser-power-resolved and temperature-resolved photocurrent measurements reveal that the photocarrier generation and recombination are dominated by continuously distributed trap states within the band gap. To understand wavelength-resolved photocurrent, we also calculated the optical absorption properties via density functional theory. Finally, we have shown that the device has a fast photoresponse with a rise time as fast as ∼1 ms. Our study provides a fundamental understanding of the optoelectronic behavior in a complex naturally occurring vdWH, and may pave an avenue toward developing nanoscale optoelectronic devices with tailored properties.

We demonstrate the first 1-transistor-1-resistor (1T1R) memory cell using the atomically thin molybdenum disulfide (MoS2) field-effect transistor (FET) and resistive random access memory (RRAM). This 1T1R demonstration realizes a key milestone for tight integration of memory with logic in a monolithic 3D integrated chip. The monolayer MoS2 is grown by chemical vapor deposition (CVD), suitable for wafer-scale fabrication. The MoS2 FETs have ON-state current of 190 μA/μm at Vd = 2.5 V, showing strong driving capability for RRAM. Metal-oxide RRAMs are fabricated at low process temperature, compatible with MoS2 FET fabrication. 1T1R measurements show higher resistances, and less resistance and voltage variation compared with measurements using only the RRAM. The multiple resistance states obtained for pulsed reset measurements show promise for in-memory computing and neuromorphic computing applications.