Tapas Pal Majumder

Ultra-thin graphene edges at the nanowire tips: a cascade cold cathode with two-stage field amplification

A multistage field emitter based on graphene-linked ZnO nanowire array is realized by means of spin-coating a graphene dispersion (reduced graphene oxide) over a nanostructured platform followed by plasma modification. Spin-coating leads to interlinking of graphene sheets between the neighboring nanowires whereas plasma etching in the subsequent step generates numerous ultra-sharp graphene edges at the nanowire tips. The inherent tendency of graphene to lay flat over a plane substrate can easily be bypassed through the currently presented nanostructure platform based technique. The turn-on and threshold field significantly downshifted compared to the individual components in the cascade emitter. Through the facile electron transfer from nanowires to graphene due to band bending at the ZnO-graphene interface together with multistage geometrical field enhancement at both the nanowire and graphene edges remain behind this enriched field emission from the composite cold cathode. This strategy will open up a new direction to integrate the functionalities of both the graphene array and several other inorganic nanostructure array for practical electronic devices.

Electro-optical and dielectric properties of a high tilt antiferroelectric liquid crystal mixture (W-193B)

The electro-optical properties and dielectric relaxation have been investigated for an antiferroelectric liquid crystal mixture W-193B. The material exhibits smectic A ∗ , smectic C ∗ and a wide range of anticlinic smectic C ∗ phases. The high tilt and broad room temperature smectic C ∗ phase make it a good candidate for antiferroelectric display materials. Dielectric studies have been made in a planarly aligned cell in the frequency range 10 Hz–13 MHz. Dielectric spectroscopy reveals the existence of soft mode in the smectic A ∗ phase and Goldstone mode in the smectic C ∗ phase. In the smectic C ∗ phase the dielectric spectrum of the material exhibits two absorption peaks related to the rotational fluctuation around the short axis of the molecules and antiphase azimuthal angle fluctuation, respectively, and are separated by about two orders of frequency. Electro-optical response using a low frequency triangular wave showed a very high quasi-static contrast ratio of 132 : 1, threshold voltage of around 7 V and saturation of 17 V. Surface-stabilized, low thickness cells of this mixture showed a perfect double hysteresis loop with a 1 Hz triangular signal, reaching different transmission levels for different voltage amplitudes. These levels can be stabilized with a single holding voltage, making it possible for the material to be passively multiplexed at video rate. (Some figures in this article are in colour only in the electronic version)

Effect of spontaneous polarization on smectic C*-smectic A* phase transition temperature and the thickness dependence of the spontaneous polarization of ferroelectric liquid crystal

The influence of spontaneous polarization on the smectic C*-smectic A* phase transition was studied theoretically by means of Landau theory. calculations that the transition temperature from a non-chiral to chiral smectic C phase is shifted due to the effect of bilinear and biquadratic couplings. A qualitative relationship has also been derived between spontaneous polarization and cell thickness which supports the experimental observations of the increase of spontaneous polarization with the increase of cell thickness for surface-stabilized ferroelectric liquid crystals. It was observed from the theoretical

Dielectric relaxation spectroscopy and electro-optical studies of a new, partially fluorinated orthoconic antiferroelectric liquid crystal material exhibiting V-shaped switching

In this paper we report the results from detailed electro-optical and dielectric studies in various antiferroelectric and ferroelectric phases of an orthoconic antiferroelectric liquid crystal (OAFLC) material . The material possesses high tilt and high spontaneous polarisation. Such an OAFLC, because of its high tilt, provides an excellent dark state. The material exhibits V-shaped switching in the SmC* phase. Dielectric studies reveal the existence of another phase during heating in the range between 78.6 and 92°C which did not appear in the DSC curve and in polarising microscopy. This phase has been identified as the SmCγ* phase and is extremely sensitive with respect to the cell conditions, aligning material, purity, etc. Three dielectric modes have been assigned in the above-mentioned temperature range and their origins are discussed.

Influence of Network Stabilization on the Dielectric and Electrooptical Properties of Ferroelectric Liquid Crystal FELIX-M4851/100

Dielectric and electrooptical properties as well as phase transition behavior in a network-stabilized ferroelectric liquid crystal were studied. The influence of polymerized network on these physical parameters were characterized. All measurements were performed upon adding monomer to the ferroelectric liquid crystal (FLC) mixture after in situ polymerization with UV radiation. We observed hysteresis-free symmetric V-shaped switching in a 2 µm cell. It was observed that both the Iso → N* and smectic A (SmA) → smectic C* (SmC*) phase transition temperatures were depressed to more ordered phase with increasing monomer concentration. The decrease was more prominent in the case of the SmA → SmC* phase transition. The Goldstone mode dielectric strength ΔeG in the SmC* phase was drastically reduced in the presence of networks. Soft mode dielectric strength was also reduced considerably. A small increase of pitch was observed with increasing monomer concentration. Spontaneous polarization was dramatically reduced in the presence of polymer networks.

Effect of Polymer Network in Polymer Dispersed Ferroelectric Liquid Crystals (PSFLC)

Polymer cross links chains can make a tremendous interaction with the ferroelectric ordering of a polymer stabilized ferroelectric liquid crystals (PSFLC). After suitable consideration of such interaction, we theoretically can study the effect of free volumes associated with the creation of bulk free energy of such PSFLC system. The variation of spontaneous polarization, tilt angle, rotational viscosity and dielectric constant depending on the assumed interactions can be demonstrated theoretically. A shift of transition temperature highly influenced by this interaction between polymer network and liquid crystal molecules was studied. By introducing the effect of cross-linked chains of polymer network we can also study the reduced behavior of the spontaneous polarization, the tilt angle and rotational viscosity with the increase of polymer concentration in the composite system. It was observed that the interacting cross-linked polymer chains played an important role in promoting multistability and resolution of the memory state in the composite system.