Khurram Shehzad

Three-dimensional macro-structures of two-dimensional nanomaterials

If two-dimensional (2D) nanomaterials are ever to be utilized as components of practical, macroscopic devices on a large scale, there is a complementary need to controllably assemble these 2D building blocks into more sophisticated and hierarchical three-dimensional (3D) architectures. Such a capability is key to design and build complex, functional devices with tailored properties. This review provides a comprehensive overview of the various experimental strategies currently used to fabricate the 3D macro-structures of 2D nanomaterials. Additionally, various approaches for the decoration of the 3D macro-structures with organic molecules, polymers, and inorganic materials are reviewed. Finally, we discuss the applications of 3D macro-structures, especially in the areas of energy, environment, sensing, and electronics, and describe the existing challenges and the outlook for this fast emerging field.

Mechanical properties of nickel-graphene composites synthesized by electrochemical deposition

Graphene (Gr) nanosheets with multilayer structures were dispersed in a nickel (Ni) plating solution by using a surfactant with a magnetic stirring method. Gr nanosheets were incorporated into a Ni matrix through a plating process to form Ni-Gr composites on a target substrate. Gr nanosheets were uniformly dispersed in the Ni matrix, and the oxygen radicals present in the Gr were reduced during the electro-deposition process. The incorporation of Gr in the Ni matrix increases both the inter-planar spacing and the degree of preferred orientation of crystalline Ni. With the addition of Gr content as low as 0.05 g L(-1), the elastic modulus and hardness of the Ni-Gr composites reach 240 GPa and 4.6 GPa, respectively, which are about 1.7 and 1.2 times that of the pure Ni deposited under the same condition. The enhancement in mechanical properties of the composites is attributed to the preferred formation of the Ni crystalline phases in its (111) plane, the high interaction between Ni and Gr and the prevention of the dislocation sliding in the Ni matrix by the Gr. The results suggest that the method of using Gr directly instead of graphene oxide (GO) is efficient and scalable.

Black phosphorus ink formulation for inkjet printing of optoelectronics and photonics.

Black phosphorus is a two-dimensional material of great interest, in part because of its high carrier mobility and thickness dependent direct bandgap. However, its instability under ambient conditions limits material deposition options for device fabrication. Here we show a black phosphorus ink that can be reliably inkjet printed, enabling scalable development of optoelectronic and photonic devices. Our binder-free ink suppresses coffee ring formation through induced recirculating Marangoni flow, and supports excellent consistency (< 2% variation) and spatial uniformity (< 3.4% variation), without substrate pre-treatment. Due to rapid ink drying (< 10 s at < 60 °C), printing causes minimal oxidation. Following encapsulation, the printed black phosphorus is stable against long-term (> 30 days) oxidation. We demonstrate printed black phosphorus as a passive switch for ultrafast lasers, stable against intense irradiation, and as a visible to near-infrared photodetector with high responsivities. Our work highlights the promise of this material as a functional ink platform for printed devices.Atomically thin black phosphorus shows promise for optoelectronics and photonics, yet its instability under environmental conditions and the lack of well-established large-area synthesis protocols hinder its applications. Here, the authors demonstrate a stable black phosphorus ink suitable for printed ultrafast lasers and photodetectors.

On Refining the Relationship between Aspect Ratio and Percolation Threshold of Practical Carbon Nanotubes/Polymer Nanocomposites

Four different aspect ratio (AR) based multiwall carbon nanotube (MWNT)/thermoplastic elastomer (TPE) polymer nanocomposites were fabricated by melt blending. In contrast to proposed mathematical models, electrical percolation thresholds of resulting nanocomposites displayed an oscillating, non-monotonic relation with increase in AR. Experimental results suggested that mathematically calculated nominal AR of MWNT was an unclear parameter to predict the percolation threshold of polymer nanocomposites. Instead, a more clear approach taking into consideration the diameter and length of nanotubes individually rather as a combined parameter of AR explained fairly well the relation between MWNT dimensions and percolation threshold.

Influence of carbon nanotube dimensions on the percolation characteristics of carbon nanotube/polymer composites

The effect of carbon nanotube aspect ratio (AR) on the percolation characteristics of their polymer composites was investigated by melt blending the multi-wall carbon nanotubes (MWCNTs) with different AR with a thermoplastic elastomer. Previously, most studies reported the effect of aspect ratio of MWCNTs only in the context of achieving the maximum electrical conductivity at lower percolation thresholds in the polymer composites. In this study, our results indicate that aspect ratio can also influence other percolation properties such as the pre-percolation conductivity, percolation conductivity and post-percolation conductivity, shape of the percolation curve, and the width of the insulator-conductor transition. We have established that AR can be used to tailor the percolation curves from sharp to quasi-linear ones, which can help us fabricate the percolative composites with stable electrical properties. Experimental results suggested that the mathematically calculated nominal AR of the MWCNTs was an un...

The effect of aspect ratio on the piezoresistive behavior of the multiwalled carbon nanotubes/thermoplastic elastomer nanocomposites

To explore the effect of aspect ratio (AR) of carbon nanotubes (CNT) on the piezoresistive behavior of the composites, four kinds of multiwalled carbon nanotubes (MWNT) with different nominal aspect ratios (AR = 62, 133, 433, and 833) were well dispersed in a thermoplastic elastomer (TPE) via melt blending. The piezoresistivity of the MWNT/TPE nanocomposites was found to be dependent on the nominal MWNT aspect ratios. However, their relationship is non-linear and non-monotonic. By introducing the effective MWNT aspect ratios which are length-dependent and diameter-dependent, it has been demonstrated that the piezoresistivity will decrease with the increase of effective aspect ratios. The length-dependent increase of aspect ratio results in one hundred-fold or more decrease of piezoresistivity, but the diameter-dependent increase of aspect ratios only leads to a slight marginal change of the piezoresistivity. The proper selection of MWNT aspect ratios could enable their utilization to tailor as well as fin...

A self-powered high-performance graphene/silicon ultraviolet photodetector with ultra-shallow junction: breaking the limit of silicon?

This work is supported by National Science Foundation (DMR1508144), NSFC (Grant Nos. 61274123, 61474099, 61674127,and 61431014), and micro-fabrication/nano-fabrication platform of ZJU University, and the Fundamental Research Funds for the Central Universities (2016XZZX001-05). This work is also supported by ZJU Cyber Scholarship and Cyrus Tang Center for Sensor Materials and Applications, the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University, the Open Research Fund of State Key Laboratory of Nanodevices and Applications at Chinese Academy of Sciences (No.14ZS01), and Visiting-by-Fellowship of Churchill College at University of Cambridge.

Facile Synthesis of γ-In2 Se3 Nanoflowers toward High Performance Self-Powered Broadband γ-In2 Se3 /Si Heterojunction Photodiode

An effective colloidal process involving the hot-injection method is developed to synthesize uniform nanoflowers consisting of 2D γ-In2 Se3 nanosheets. By exploiting the narrow direct bandgap and high absorption coefficient in the visible light range of In2 Se3 , a high-quality γ-In2 Se3 /Si heterojunction photodiode is fabricated. This photodiode shows a high photoresponse under light illumination, short response/recovery times, and long-term durability. In addition, the γ-In2 Se3 /Si heterojunction photodiode is self-powered and displays a broadband spectral response ranging from UV to IR with a high responsivity and detectivity. These excellent performances make the γ-In2 Se3 /Si heterojunction very interesting as highly efficient photodetectors.

Two percolation thresholds and remarkably high dielectric permittivity in pristine carbon nanotube/elastomer composites

Pristine carbon nanotube (CNT)/elastomer composites were fabricated using pristine multi-walled carbon nanotubes and a thermoplastic elastomer. These composites exhibited a unique phenomenon of two electrical percolation thresholds that invoked very high dielectric values for the resulting composites. The first percolation was associated with a relatively low dielectric constant value of about 100, while in the vicinity of the second percolation threshold a very high dielectric constant value of 8,000 was achieved. The presence of two percolation thresholds was attributed to the unique distribution patterns of CNTs that ensued in a CNT/elastomer composite system with unique electrical properties.

A Broadband Fluorographene Photodetector

High-performance photodetectors operating over a broad wavelength range from ultraviolet, visible, to infrared are of scientific and technological importance for a wide range of applications. Here, a photodetector based on van der Waals heterostructures of graphene and its fluorine-functionalized derivative is presented. It consistently shows broadband photoresponse from the ultraviolet (255 nm) to the mid-infrared (4.3 µm) wavelengths, with three orders of magnitude enhanced responsivity compared to pristine graphene photodetectors. The broadband photodetection is attributed to the synergistic effects of the spatial nonuniform collective quantum confinement of sp2 domains, and the trapping of photoexcited charge carriers in the localized states in sp3 domains. Tunable photoresponse is achieved by controlling the nature of sp3 sites and the size and fraction of sp3 /sp2 domains. In addition, the photoresponse due to the different photoexcited-charge-carrier trapping times in sp2 and sp3 nanodomains is determined. The proposed scheme paves the way toward implementing high-performance broadband graphene-based photodetectors.