Water-assisted controllable growth of atomically thin WTe2 nanoflakes by chemical vapor deposition based on precursor design and substrate engineering strategies

WTe2 nanostructures have intrigued much attention due to their unique properties, such as large non-saturating magnetoresistance, quantum spin Hall effect and topological surface state. However, the controllable growth of large-area atomically thin WTe2 nanostructures remains a significant challenge. In the present work, we demonstrate the controllable synthesis of 1T’ atomically thin WTe2 nanoflakes (NFs) by water-assisted ambient pressure chemical vapor deposition method based on precursor design and substrate engineering strategies. The introduction of water during the growth process can generate a new synthesized route by reacting with WO3 to form intermediate volatile metal oxyhydroxide. Using WO3 foil as the growth precursor can drastically enhance the uniformity of as-prepared large-area 1T’ WTe2 NFs compared to WO3 powders. Moreover, highly oriented WTe2 NFs with distinct orientations can be obtained by using a-plane and c-plane sapphire substrates, respectively. Corresponding precursor design and substrate engineering strategies are expected to be applicable to other low dimensional transition metal dichalcogenides, which are crucial for the design of novel electronic and optoelectronic devices.

Hot Carrier Transfer in graphene/PtSe2 Heterostructure Tuned by Built-in Electric Field

<p>Van der Waals heterojunction involving graphene (Gr) with transition metal dichalcogenides (TMDs) is regard as a promising structure for their outstanding performance in optical and optoelectronic response. The electron-hole thermalization has been deemed to be the main reason for the sub-bandgap-excitation charge transfer from Gr to TMDs. However, the role of the intricate interlayer interaction of the Gr and the TMDs still require intensive investigation. Here, we have investigated the photocarrier dynamics in 5-layer PtSe₂/Gr heterojunction by using time-resolved optical pump and terahertz probe spectroscopy. Interestingly, after photoexcitation, electron transfer from PtSe₂ to Gr in PtSe₂/Gr/substrate heterojunction has been demonstrated successfully, by contrast, no observable charge transfer occurs in the Gr/PtSe₂/substrate heterostructure. The prominent difference for the different stacking sequence between Gr and PtSe₂ can be ascribed to the effective built-in field introduced by fused silica substrate. A physical picture accounting for built-in electric field introduced by substrate has been proposed to interpret the charge transfer process in the TMD/Gr heterostructure–the substrate built-in electric field plays a dominated role for controlling the charge transfer pathway in the TMDs/Gr heterojunction. This study not only shed the light to the substrate engineering but also provide a new insight into the dynamic in Gr/TMDs heterojunction, which provides a new method to optimize the performance of photodetection. </p>

Hot Carrier Transfer in graphene/PtSe2 Heterostructure Tuned by Built-in Electric Field

<p>Van der Waals heterojunction involving graphene (Gr) with transition metal dichalcogenides (TMDs) is regard as a promising structure for their outstanding performance in optical and optoelectronic response. The electron-hole thermalization has been deemed to be the main reason for the sub-bandgap-excitation charge transfer from Gr to TMDs. However, the role of the intricate interlayer interaction of the Gr and the TMDs still require intensive investigation. Here, we have investigated the photocarrier dynamics in 5-layer PtSe₂/Gr heterojunction by using time-resolved optical pump and terahertz probe spectroscopy. Interestingly, after photoexcitation, electron transfer from PtSe₂ to Gr in PtSe₂/Gr/substrate heterojunction has been demonstrated successfully, by contrast, no observable charge transfer occurs in the Gr/PtSe₂/substrate heterostructure. The prominent difference for the different stacking sequence between Gr and PtSe₂ can be ascribed to the effective built-in field introduced by fused silica substrate. A physical picture accounting for built-in electric field introduced by substrate has been proposed to interpret the charge transfer process in the TMD/Gr heterostructure–the substrate built-in electric field plays a dominated role for controlling the charge transfer pathway in the TMDs/Gr heterojunction. This study not only shed the light to the substrate engineering but also provide a new insight into the dynamic in Gr/TMDs heterojunction, which provides a new method to optimize the performance of photodetection. </p>

Biocatalytic Reduction of Activated Cinnamic Acid Derivatives : Asymmetric reduction of C=C double bonds using Johnson Matthey enzymes

The asymmetric reduction of C=C double bonds is a sought-after chemical transformation to obtain chiral molecules used in the synthesis of fine chemicals. Biocatalytic C=C double bond reduction is a particularly interesting transformation complementary to more established chemocatalytic methods. The enzymes capable of catalysing this reaction are called ene-reductases (ENEs). For the reaction to take place, ENEs need an electron withdrawing group (EWG) in conjugation with the double bond. Especially favourable EWGs are carbonyls and nitro groups; other EWGs, such as carboxylic acids, esters or nitriles, often give poor results. In this work, a substrate engineering strategy is proposed whereby a simple transformation of the carboxylic acid into a fluorinated ester or a cyclic imide allows to increase the ability of ENEs to reduce the conjugated double bond. Up to complete conversion of the substrates tested was observed with enzymes ENE-105 and *ENE-69.