1.  Stretchable and Multimodal All Graphene Electronic Skin (by Dong Hae HO)

We demonstrated all graphene–based transparent and conformable multifunctional E–skin matrix. The CVD–Gr was used as electrodes in the matrix, while GO and rGO were adopted as sensing materials. A simple lamination process dexterously integrated the humidity, temperature, and pressure sensors as a whole. Each sensor was sensitive to its relevant external stimulus, but not affected by the other two stimuli.

Moreover, all the sensors could work simultaneously and indicate different stimulations individually. The distributions of the temperature, humidity, and pressure under figure pressing were represented in 2D color mapping. This work suggested a facile fabrication process combined with graphene derivatives to transparent and conformable E–skin application, which overcome the conventional complex E–skin fabrication process. Additional sensors and wireless communication units could also be integrated through this simple lamination process, which would greatly help to realize interactive and remote health care in the future. Utilization of various graphene derivatives as main components in E–skin could also open up a way to accelerate the industrialization of graphene.

Link: http://onlinelibrary.wiley.com/wol1/doi/10.1002/adma.201505739/full

2.  Low-Voltage Complementary Electronics from Ion-Gel-Gated Vertical Van der Waals Heterostructures (by Yong Suk CHOI)

 Isolation of ultrathin layered materials with disparate electronic properties has prompted their integration into van der Waals heterostructures with unique properties for electronics. By employing ion gel dielectrics, we demonstrate here low-voltage complementary circuits comprised of n-type and p-type van der Waals heterojunction vertical field-effect transistors (VFETs). The n-type and p-type VFETs were fabricated from MoS₂ and WSe₂ layers on graphene, respectively, with gating achieved by a top ion gel dielectric. Due to the high specific capacitance of the ion gel dielectric, the work function of the underlying graphene and consequently the energy barrier for charge injection into the MoS₂ and WSe₂ layers, are widely tuned at low operating voltages. The resulting VFETs possess high on-state current densities (> 3000 A cm^-2) and on/off current ratios (> 10⁴) in a narrow voltage window (< 3 V). In addition to enhancing VFET device metrics at reduced operating voltages, the high specific capacitance of the ion gel dielectric allows unprecedented regimes of charge transport to be accessed including p-channel phenomena in graphene-MoS₂ vertical heterostructures.

Furthermore, since the ion gel-gated VFETs possess low threshold voltages, a low-power complementary logic inverter is demonstrated using n-type and p-type VFETs.

Overall, these results demonstrate the feasibility of vertical van der Waals heterostructures for high-performance, low-voltage electronics.

Link: http://onlinelibrary.wiley.com/wol1/doi/10.1002/adma.201506450/full

3.  An Organic Vertical Field-Effect Transistor with Underside-Doped Graphene Electrodes (by Jong Su KIM)

We fabricated vertical Schottky barrier (SB) transistors based on graphene–organic semiconductor (p-type pentacene or n-type N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8)) heterostructures. The Schottky barrier formed at the vertical graphene-organic semiconductor heterojunction could be modulated by tuning the applied gate voltage. Interestingly, molecular dopants (p-type bis-(trifluoromethane sulfonyl) amine (TFSA) or n-type poly (ethylene imine) were inserted underneath the graphene layer, which we refer to as the underside-doping method. This doping method enabled tuning the work function of the graphene while maintaining the surface properties of the graphene electrodes at the graphene-organic heterojunction, unlike when the dopants were placed on top of the graphene electrode (i.e. the topside-doping method). This charge injection at the heterojunction as well as the vertical transistor performances, including the device current density and on-off current ratio, varied systematically with doping the graphene electrode. The optimized p- and n-type devices yielded a high current density of 11 mA·cm^–2 and a high on-off current ratio of ~10³. Complementary inverters were successfully fabricated by assembling the p-type and n-type vertical SB transistors. The proposed underside-doping method opens up new opportunities for realizing future organic electronics.

Link: http://onlinelibrary.wiley.com/wol1/doi/10.1002/adma.201505378/full

4.  Organic Dye Graphene Hybrid Structures with Spectral Color Selectivity (by Yu Sung GIM)

This study characterizes a hybrid structure formed between graphene and organic dye molecules for use in photodetectors with spectral color selectivity. Rhodamine-based organic dye molecules with red, green, or blue light absorption profiles are deposited onto a graphene surface by dip-coating. UV–vis absorption spectroscopy, charge transport measurements, and density functional theory based calculations reveal that the photoresponses of the dye graphene hybrid films are governed by the light absorption of the dye molecules and also by the photo-excited-charge-transfer-induced photocurrent gain. The hybrid films respond only to photons with an energy exceeding the band gap of the immobilized dye. Dye-Graphene charge transfer is affected by the distance and direction of the dipole moment between the two layers. The resulting hybrid films exhibit spectral color selectivities with responsivities of ≈10³ A W⁻¹ and specific detectivities of ≈10¹⁰ Jones.

This study demonstrates the successful operation of photodetectors with a full-color optical bandwidth using hybrid graphene structures coated with a mixture of dyes. The strategy of building a simple hybrid photodetector can further offer many opportunities to be also tuned for other optical functionalities using a variety of commercially available dye molecules.

Link: http://onlinelibrary.wiley.com/doi/10.1002/adfm.201601200/full

5.  Mechanically Robust Silver Nanowires Network for Triboelectric Nanogenerators (by Hyung Seok KANG)

The authors develop a mechanically robust silver nanowires (AgNWs) electrode platform for use in flexible and stretchable triboelectric nanogenerators (TENGs). The embedding of an AgNWs network into a photocurable or thermocurable polymeric matrix dramatically enhances the mechanical robustness of the flexible and stretchable TENG electrodes while maintaining a highly efficient triboelectric performance. The AgNWs/polymeric matrix electrode is fabricated in four steps: (i) the AgNWs networks are formed on a hydrophobic glass substrate; (ii) a laminating photocurable or thermocurable prepolymer film is applied to the developed AgNWs network; (iii) the polymeric matrix is crosslinked by UV exposure or thermal treatment; and (iv) the AgNWs-embedded polymeric matrix is delaminated from the glass substrate. The AgNWs-embedded polymeric matrix electrodes with four different sheet resistances, controlled by varying the AgNWs network deposition density, are deployed in TENG devices. The authors find that the potential difference between the two contact surfaces of the AgNWs network-embedded polymer matrix electrodes and the nylon (or perfluoroalkoxy alkane) governs the output triboelectric performances of the devices, rather than the sheet resistance. Both Kelvin probe force microscopy and numerical simulations strongly support these observations.

Link: http://onlinelibrary.wiley.com/wol1/doi/10.1002/adfm.201603199/full

6.  Multibit MoS₂ Photoelectronic Memory with Ultrahigh Sensitivity (by Da In LEE)

Taking advantage of the superlative optoelectronic properties of single-layer MoS₂, we developed novel MoS₂ optoelectronic memory devices in which single-layer MoS₂ films with a direct band gap of 1.8 eV were utilized as both the channel material and the light absorption layer. The advanced transfer method, benefitted from the adhesion-strained metallic layer, allowed us to obtain large-area high-quality single-layer MoS₂ flakes that provided a phototransistor with ultrahigh sensitivity that exhibited a photoresponsivity of 8,024 AW^–1 and a photodetectivity of 10¹² Jones at an illumination power of 0.1 μW.

In addition to the ultrahigh photosensitivity, the device memorized the number of incident photons in the form of a persistent current if gold nanoparticles (AuNPs) were used as the floating gates. Electron transfer from the AuNPs, along with the Pauli blocking mechanism, prevented the photoexcited electron–hole pairs from recombining; therefore, the persistent current was proportional to the intensity of light incident on the device. Multilevel data storage of the incident photons could be modulated precisely by tuning the applied gate voltage and the photo-illumination power. The MoS₂ photonic memory exhibited excellent memory characteristics, including a large programing/erasing current ratio that exceeded 10⁷, multilevel data storage of 3 bits (corresponding to 8 levels), performance stability over 200 cycles, and stable data retention over 10⁴ s. 6.

Link: http://onlinelibrary.wiley.com/doi/10.1002/adma.201603571/full

7.  Piezopotential-Programmed Multilevel Nonvolatile Memory As Triggered by Mechanical Stimuli (by Qijun SUN)

We report the development of a piezopotential-programmed nonvolatile memory array using a combination of ion gel-gated field-effect transistors (FETs) and piezoelectric nanogenerators (NGs). Piezopotentials produced from the NGs under external strains were able to replace the gate voltage inputs associated with the programming/erasing operation of the memory, which reduced the power consumption compared with conventional memory devices. Multilevel data storage in the memory device could be achieved by varying the external bending strain applied to the piezoelectric NGs. The resulting devices exhibited good memory performance, including a large programming/erasing current ratio that exceeded 10³, multilevel data storage of 2 bits (over 4 levels), performance stability over 100 cycles, and stable data retention over 3000 s. The piezopotential-programmed multilevel nonvolatile memory device described here is important for applications in data-storable electronic skin and advanced human-robot interface operations.

Link: http://pubs.acs.org/doi/abs/10.1021/acsnano.6b05895