Stephan Kraemer

A subthermionic tunnel field-effect transistor with an atomically thin channel.

The fast growth of information technology has been sustained by continuous scaling down of the silicon-based metal–oxide field-effect transistor. However, such technology faces two major challenges to further scaling. First, the device electrostatics (the ability of the transistor’s gate electrode to control its channel potential) are degraded when the channel length is decreased, using conventional bulk materials such as silicon as the channel. Recently, two-dimensional semiconducting materials have emerged as promising candidates to replace silicon, as they can maintain excellent device electrostatics even at much reduced channel lengths. The second, more severe, challenge is that the supply voltage can no longer be scaled down by the same factor as the transistor dimensions because of the fundamental thermionic limitation of the steepness of turn-on characteristics, or subthreshold swing. To enable scaling to continue without a power penalty, a different transistor mechanism is required to obtain subthermionic subthreshold swing, such as band-to-band tunnelling. Here we demonstrate band-to-band tunnel field-effect transistors (tunnel-FETs), based on a two-dimensional semiconductor, that exhibit steep turn-on; subthreshold swing is a minimum of 3.9 millivolts per decade and an average of 31.1 millivolts per decade for four decades of drain current at room temperature. By using highly doped germanium as the source and atomically thin molybdenum disulfide as the channel, a vertical heterostructure is built with excellent electrostatics, a strain-free heterointerface, a low tunnelling barrier, and a large tunnelling area. Our atomically thin and layered semiconducting-channel tunnel-FET (ATLAS-TFET) is the only planar architecture tunnel-FET to achieve subthermionic subthreshold swing over four decades of drain current, as recommended in ref. 17, and is also the only tunnel-FET (in any architecture) to achieve this at a low power-supply voltage of 0.1 volts. Our device is at present the thinnest-channel subthermionic transistor, and has the potential to open up new avenues for ultra-dense and low-power integrated circuits, as well as for ultra-sensitive biosensors and gas sensors.

A Facile Synthesis of Dynamic, Shape‐Changing Polymer Particles

We herein report a new facile strategy to ellipsoidal block copolymer nanoparticles that exhibit a pH-triggered anistropic swelling profile. In a first step, elongated particles with an axially stacked lamellae structure are selectively prepared by utilizing functional surfactants to control the phase separation of symmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) in dispersed droplets. In a second step, the dynamic shape change is realized by cross-linking the P2VP domains, thereby connecting glassy PS discs with pH-sensitive hydrogel actuators.

Photoelectrochemical Performance of CdSe Nanorod Arrays Grown on a Transparent Conducting Substrate

Perpendicularly aligned semiconducting CdSe nanorod arrays were fabricated on ITO-coated glass substrate using porous aluminum oxide (PAO) as a hard template. Nanorod lengths were varied between 50 and 500 nm, while keeping the diameter at 65 nm. The electrochemical photovoltaic performance was found to depend critically on nanorod length and crystallinity. Arrays of rods annealed at 500 degrees C showed an order of magnitude improvement in white light power conversion efficiency over unannealed samples. The largest power conversion efficiency of 0.52% was observed for nanorods 445 +/- 82 nm in length annealed at 500 degrees C. The technique described is generally applicable to fabricating highly aligned nanorods of a broad range of materials on a robust transparent conductor.

Patterned Formation of Highly Coherent Nitrogen-Vacancy Centers Using a Focused Electron Irradiation Technique

We demonstrate fully three-dimensional and patterned localization of nitrogen-vacancy (NV) centers in diamond with coherence times in excess of 1 ms. Nitrogen δ-doping during chemical vapor deposition diamond growth vertically confines nitrogen to 4 nm while electron irradiation with a transmission electron microscope laterally confines vacancies to less than 450 nm. We characterize the effects of electron energy and dose on NV formation. Importantly, our technique enables the formation of reliably high-quality NV centers inside diamond nanostructures with applications in quantum information and sensing.

Highly functional ellipsoidal block copolymer nanoparticles: a generalized approach to nanostructured chemical ordering in phase separated colloidal particles

A new synthetic platform for the spatially controlled functionalization of phase separated block copolymer nanoparticles is presented. Selective incorporation of chemical functionalities into specific domains of striped ellipsoidal nanoparticles is achieved by blending a structure-inducing PS-b-P2VP block copolymer with functionalized PS-co-X and P2VP-co-X copolymers. During self-assembly, the BCP phases incorporate the corresponding functional copolymers which results in their chemical modification without losing control over particle shape and morphology. It was shown that the introduction of benzophenones as photocrosslinking groups allows the preparation of particles which demonstrated a reversible shape change due to triggered swelling/deswelling. This dynamic behavior could be combined with the selective introduction of other moieties such as ferrocene groups or reactive pentafluorostyrene moieties. Ultimately, such combinations opened up new opportunities for post-assembly functionalizations to realize multifunctional particles containing for example ferrocene moieties in the PS domains and Au nanoparticles in the P2VP phase. Overall, a versatile toolbox was developed that enables the formation of tailor-made functional shape anisotropic nanoparticles.

A molecular cross-linking approach for hybrid metal oxides

There is significant interest in the development of methods to create hybrid materials that transform capabilities, in particular for Earth-abundant metal oxides, such as TiO2, to give improved or new properties relevant to a broad spectrum of applications. Here we introduce an approach we refer to as ‘molecular cross-linking’, whereby a hybrid molecular boron oxide material is formed from polyhedral boron-cluster precursors of the type [B12(OH)12]2–. This new approach is enabled by the inherent robustness of the boron-cluster molecular building block, which is compatible with the harsh thermal and oxidizing conditions that are necessary for the synthesis of many metal oxides. In this work, using a battery of experimental techniques and materials simulation, we show how this material can be interfaced successfully with TiO2 and other metal oxides to give boron-rich hybrid materials with intriguing photophysical and electrochemical properties.TiO2 and other metal oxides were interfaced with molecular boron clusters to form a hybrid material. This modifies the electrochemical and photocatalytic properties, enabling fast electron transfer and dye degradation under red light.

12 nm-gate-length ultrathin-body InGaAs/InAs MOSFETs with 8.3•10 5 I ON /I OFF

We report III-V MOSFETs with 12 nm physical gate length, ultrathin 1.5/1 nm InGaAs/lnAs composite channels, and recessed doping-graded InP SID vertical spacers. The FETs demonstrate g_m-1.8 mS/μm transconductance, SS-107 mV/dec., minimum loJj1.3 nA/μm at V_DS=0.5 V, and well-balanced on-off DC performance with maximum I_on/l_off-8.3x 10⁵. Band-to-band tunneling leakage current is well-controlled through the thin composite InGaAs/lnAs channel, and by the recessed InP source/drain spacers. This work demonstrates that 111-V MOSFETs can scale to the sub-10-nm technology nodes.

Publisher Correction: A molecular cross-linking approach for hybrid metal oxides

In the version of this Article originally published, Liban M. A. Saleh was incorrectly listed as Liban A. M. Saleh due to a technical error. This has now been amended in all online versions of the Article.