Hossein Taghinejad

Cell-imprinted substrates act as an artificial niche for skin regeneration.

Bioinspired materials can mimic the stem cell environment and modulate stem cell differentiation and proliferation. In this study, biomimetic micro/nanoenvironments were fabricated by cell-imprinted substrates based on mature human keratinocyte morphological templates. The data obtained from atomic force microscopy and field emission scanning electron microscopy revealed that the keratinocyte-cell-imprinted poly(dimethylsiloxane) casting procedure could imitate the surface morphology of the plasma membrane, ranging from the nanoscale to the macroscale, which may provide the required topographical cell fingerprints to induce differentiation. Gene expression levels of the genes analyzed (involucrin, collagen type I, and keratin 10) together with protein expression data showed that human adipose-derived stem cells (ADSCs) seeded on these cell-imprinted substrates were driven to adopt the specific shape and characteristics of keratinocytes. The observed morphology of the ADSCs grown on the keratinocyte casts was noticeably different from that of stem cells cultivated on the stem-cell-imprinted substrates. Since the shape and geometry of the nucleus could potentially alter the gene expression, we used molecular dynamics to probe the effect of the confining geometry on the chain arrangement of simulated chromatin fibers in the nuclei. The results obtained suggested that induction of mature cell shapes onto stem cells can influence nucleus deformation of the stem cells followed by regulation of target genes. This might pave the way for a reliable, efficient, and cheap approach of controlling stem cell differentiation toward skin cells for wound healing applications.

Flexible MoS2 Field-Effect Transistors for Gate-Tunable Piezoresistive Strain Sensors

Atomically thin molybdenum disulfide (MoS2) is a promising two-dimensional semiconductor for high-performance flexible electronics, sensors, transducers, and energy conversion. Here, piezoresistive strain sensing with flexible MoS2 field-effect transistors (FETs) made from highly uniform large-area films is demonstrated. The origin of the piezoresistivity in MoS2 is the strain-induced band gap change, which is confirmed by optical reflection spectroscopy. In addition, the sensitivity to strain can be tuned by more than 1 order of magnitude by adjusting the Fermi level via gate biasing.

A Nickel–Gold Bilayer Catalyst Engineering Technique for Self-Assembled Growth of Highly Ordered Silicon Nanotubes (SiNT)

We report the growth of vertically aligned high-crystallinity silicon nanotube (SiNT) arrays on silicon substrate by means of a Ni-Au bilayer catalyst engineering technique. Nanotubes were synthesized through solid-liquid-solid method as well as vapor-liquid-solid. A precise evaluation utilizing atomic force microscopy and lateral force microscopy describes that the gold profile in Ni regions leads to the construction of multiwall SiNTs. The agreement of the structural geometry and stiffness of the obtained SiNTs with previous theoretical predictions suggest sp(3) hybridization as the mechanism of tube formation. Apart from scanning electron and transmission electron microscopy techniques, photoluminescence spectroscopy (PL) has been conducted to investigate the formation of nanostructures. PL spectroscopy confirms the evolution of ultrafine walls of the silicon nanotubes, responsible for the observed photoemission properties.

Cell membrane electrical charge investigations by silicon nanowires incorporated field effect transistor (SiNWFET) suitable in cancer research

Obtaining biocompatible skein shaped silicon nanowires covered field-effect transistors (SiNWFETs) is reported. Such structures are formed by the growth of Si nanowires on top of the polysilicon gate terminal of a n-MOSFET in three-probe top-gate geometry for cells membrane electrical charge detection suitable in biological applications such as cancer investigations. We observe that electro-negativity increment of metastatic cells outer membrane restricts the drain current of bioFET. SiNWFET is proposed to be used covered with skein SiNW. Different invasive grades of colon cancers (HT29 and SW48) owing to their dissimilar negative charge production, affect the electrical response of the device.

Integration of Ni 2 Si/Si Nanograss Heterojunction on n-MOSFET to Realize High-Sensitivity Phototransistors

We report a top-down fabrication technique for direct integration of Ni2Si/Si heterojunction arrays on n-type MOSFET gate terminal to realize sensitive field-effect phototransistors (PTs). It was observed that exposing gate area to light (λ = 655 nm) leads to significant modulation of threshold voltage (VTH) of PT in comparison with a conventional MOSFET. By analyzing optical absorption spectra of Ni2Si/Si nanostructures on SiO2 of this novel PT and planar Ni2Si/poly-Si on SiO2 of conventional transistors, we concluded that, nanostructures strongly improve light absorption. This results in higher optically generated electron-hole pairs in which excess holes induce an electric field on the transistor channel and help the threshold happen at lower voltages. To recognize the appropriate operating point of such nanostructure-based PT, effect of VGS on the sensitivity of drain current was investigated. Experimental results show that device sensitivity (S) is related to gate-source voltage via a homographic like function. Theoretical analysis shows S ∝ 1/(VGS - VTH) for high VGS, which has been confirmed by experimental data. In addition, external quantum efficiency and photoresponsivity of PT as functions of gate voltage were studied. The achieved results suggest that this device would be a promising candidate for fabricating low-power PTs based on silicide nanostructures.

Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys

Composition modulation of two-dimensional transition-metal dichalcogenides (TMDs) has introduced an enticing prospect for the synthesis of Van der Waals alloys and lateral heterostructures with tunable optoelectronic properties. Phenomenologically, the optoelectronic properties of alloys are entangled to a strain that is intrinsic to synthesis processes. Here, we report an unprecedented biaxial strain that stems from the composition modulation of monolayer TMD alloys (e.g., MoS2xSe2(1 - x)) and inflicts fracture on the crystals. We find that the starting crystal (MoSe2) fails to adjust its lattice constant as the atoms of the host crystal (selenium) are replaced by foreign atoms (sulfur) during the alloying process. Thus, the resulting alloy forms a stretched lattice and experiences a large biaxial tensile strain. Our experiments show that the biaxial strain relaxes via formation of cracks in interior crystal domains or through less constraint bounds at the edge of the monolayer alloys. Griffith’s criterion suggests that defects combined with a sulfur-rich environment have the potential to significantly reduce the critical strain at which cracking occurs. Our calculations demonstrate a substantial reduction in fracture-inducing critical strain from 11% (in standard TMD crystals) to a range below 4% in as-synthesized alloys.2D alloys: intrinsic strain in MoS 2x Se 2(1-x) ternary crystalsComposition modulation synthesis of ternary alloys of atomically thin transition metal dichalcogenides gives rise to intrinsic biaxial strain. A team led by Ali Adibi at Georgia Institute of Technology reported the onset of a substantial biaxial strain in monolayer MoS2xSe2(1-x) that is intrinsically linked to the two-step composition modulation synthesis used to grow the ternary alloy. As the S atoms replace the Se atoms of the starting MoSe2 host crystal, the resulting alloy forms a stretched lattice and develops a large biaxial tensile strain. Morphological and spectroscopic characterisations suggest that such strain results in the onset of fracture in the crystal, and further relaxes via formation of cracks within the crystal domains. Theoretical modelling indicates that pre-existing cracks give a substantial contribution in weakening the strength of the synthesized van der Waals alloy.

Reconfigurable metasurfaces in a hybrid material platform through integration of plasmonic nanostructures with phase-change materials (Conference Presentation)

We demonstrate a highly-integrated, subwavelength, and reconfigurable nanoscale spatial light modulator capable of modulating the amplitude, phase, and polarization of impinging light both spatially and spectrally. These properties are enabled by integration of plasmonic metasurfaces with phase-change materials. Owing to the ultrafast switching speed, considerable scalability, high switching robustness, good thermal stability, adaptability with complementary metal oxide semiconductor (CMOS) technology, and large refractive index change contrast between its amorphous and crystalline phases, germanium antimony telluride (GST), a well-known PCM, is used for these miniaturized dynamic metadevices. To show the unprecedented capability of this hybrid plasmonic-PCM material platform for practical applications, we investigate a plasmonic-GST gradient metasurface comprising of a patterned array of gold nanostrips that is separated from an underlying reflecting gold plate by a thin layer of GST. While the plasmonic inclusions support enhanced short-range surface plasmons, which are highly coupled to both the electric and magnetic components of the incident optical field, the real-time structural transition between the states of GST constituent, upon excitation with an external electric stimulus, provides a remarkable refractive index contrast for reconfiguration. Such dynamic metasurfaces could lead to new avenues for realization of reconfigurable, fast, and energy-efficient miniaturized photonic components such as multifocus lenses, all-optical switches, vortex beam generator, and grayscale holograms in a reversible and nonvolatile fashion.

Alloying-induced biaxial strain in ternary alloys of transition-metal dichalcogenides (TMDs) (Conference Presentation)

Alloying has served as a powerful means for tuning the non-vanishing optical bandgap of two-dimensional (2D) transition-metal dichalcogenides (TMDs), a family of 2D materials with optoelectronic properties covering a wide spectral window ranging from visible to near-infrared. In addition to the bandgap engineering, ‘spatial’ modulation of the composition ratio (i.e., x) in a ternary TMD alloy (e.g., MX2xX2(1-x)’; M: transition metal, X, X’: chalcogens) enables formation of lateral heterostructures with complex functionalities within the plane of 2D materials, a new asset that expands the realm of applications in which 2D materials can be incorporated. Despite several demonstrations of alloying in 2D TMDs, the phenomenologically important issue of strain development and its effect on the optical and structural properties of 2D TMD alloys is still missing. Here, we show that alloying processes induce a biaxial tensile strain that acts on the lattice of 2D TMD alloys and affect their optical properties. In addition, we show that such strain inflicts sever fracture of the alloys via formation of sub-micron-sized cracks. Our experimental characterization combined with detailed theoretical modeling suggest the important role of the Van der Waals interaction between the 2D material and the substrate in formation of the alloying-induced strain. Furthermore, we demonstrate the critical role of crystal defects in cracking of the TMD alloys, which further emphasizes the importance of high quality synthesis of 2D TMD crystals for practical applications.

Ultrafast Control of Phase and Polarization of Light Expedited by Hot-Electron Transfer

All-optical modulation is an entangled part of ultrafast nonlinear optics with promising impacts on tunable optical devices in the future. Current advancements in all-optical control predominantly offer modulation by means of altering light intensity, while the ultrafast manipulation of other attributes of light have yet to be further explored. Here, we demonstrate the active modulation of the phase, polarization, and amplitude of light through the nonlinear modification of the optical response of a plasmonic crystal that supports subradiant, high Q, and polarization-selective resonance modes. The designed mode is exclusively accessible via TM-polarized light, which enables significant phase modulation and polarization conversion within the visible spectrum. To tailor the device performance in the time domain, we exploit the ultrafast transport dynamics of hot electrons at the interface of plasmonic metals and charge acceptor materials to facilitate an ultrafast switching speed. In addition, the operating wavelength of the proposed device can be tuned through the control of the in-plane momentum of light. Our work reveals the viability of dynamic phase and polarization control in plasmonic systems for all-optical switching and data processing.

Hot‐Electron‐Assisted Femtosecond All‐Optical Modulation in Plasmonics

The optical Kerr nonlinearity of plasmonic metals provides enticing prospects for developing reconfigurable and ultracompact all-optical modulators. In nanostructured metals, the coherent coupling of light energy to plasmon resonances creates a nonequilibrium electron distribution at an elevated electron temperature that gives rise to significant Kerr optical nonlinearities. Although enhanced nonlinear responses of metals facilitate the realization of efficient modulation devices, the intrinsically slow relaxation dynamics of the photoexcited carriers, primarily governed by electron-phonon interactions, impedes ultrafast all-optical modulation. Here, femtosecond (≈190 fs) all-optical modulation in plasmonic systems via the activation of relaxation pathways for hot electrons at the interface of metals and electron acceptor materials, following an on-resonance excitation of subradiant lattice plasmon modes, is demonstrated. Both the relaxation kinetics and the optical nonlinearity can be actively tuned by leveraging the spectral response of the plasmonic design in the linear regime. The findings offer an opportunity to exploit hot-electron-induced nonlinearities for design of self-contained, ultrafast, and low-power all-optical modulators based on plasmonic platforms.