PacSurf2016 Session NM-TuP: Nanomaterials Poster Session

As is well known, vicinal superlattices (VSLs) are realized in 2D electron systems on semiconductor high-index Miller surfaces. The possibility of existence of such VSLs was predicted theoretically (V.A.Petrov, 1977) ^1,2; simultaneously and independently they were realized (T.Cole et al.1977) ³. At the present time, all these VSLs are developed only in 2D systems.

In this work we suggest a new method of development of the VSL in the quantum wires (QWR).where superlattice effects should be maximal. The special feature of this method is the combination of the main properties of the VSL - the separation in the system by some way of the long translation period A - with the possibility of developing this situation in the QWR on semiconductor low - index surfaces. It is easy to see that this situation is possible when the axis of the QWR which lies on the low - index surface will be oriented at the necessary angles to the basic translation vectors along the surface. In this case the translation symmetry of the QWR will be determined by its orientation on the crystal surface since the possibility of a free motion only along the axis of the wire selects in the initial two - dimensional translation group along the surface a one - dimensional translation subgroup along the wire with the basic period A. Thus, in the one-dimensional VSL the period A is selected by the orientation of the wire on the surface. For example, if the QWR is realized in the MOS system with the use of a narrow gate (V.A.Petrov 1978)⁴ then the orientation of the wire will be determined simply by the appropriate orientation of the gate.

The analytic expressions of the new periods A were obtained as a function of the angles which determine orientation of the QWR for the different low-index surfaces GaAs and Si. The positions of minigaps in the one-dimensional k-space were determined. It is should be noted that in the region of the particle wave function localization in the QWR there are many crystallographic planes which form a superlattice energetic spectrum of the particle. Illustrative estimates of the magnitude of the minigaps for the QWR of the rectangular cross-section made in the weak coupling approximation demonstrate their dependence on the geometric parameters of the cross-section, on the period A as well as on the crystal potential.

1.V.A.Petrov, 6th All-Union Conf.on the Physics of Surface Phenom. in Semicond. 1977,Kiev, Abstracts of Paper, Kiev,1977, part 2, p.80;

2. V. A. Petrov, Sov. Phys. Semicond.12, pp.219-220, (1978)

3. T. Cole, A. A. Lakhani and P. J. Stiles, Phys. Rev. Lett. 38, pp. 722-725, (1977)

4. V. A. Petrov, Sov. Tech. Phys. Lett 4, pp.285-286, (1978)

The influence of the interference of electron waves in the case of their reflection from potential barrier on the spatial distribution of the density of quantum-mechanical current ej_х(x,z) (e – electron charge) in 2D semiconductor nanostructure which is represented by rectangular narrow ( x < 0,QW₁) and wide (x > 0, QW₂) quantum wells ( QWs) sequentially oriented along the direction of the propagation of electron wave has been studied theoretically. It is supposed that the wave falls from the narrow QW₁ on the semi-infinite potential barrier V₀ in height in the wide QW₂, the energy of the falling wave being less than V₀. Differing widths of QW₁ and QW₂ provide the non-orthogonality of wave functions of particles in these regions and the corresponding existence of electron interferential effects in this kind of nanostructure. In particular cases these effects lead to the appearance of spatially inhomogeneous distributions еj_x⁽¹⁾(x,z) in QW₁ and еj_x⁽²⁾(x,z) in QW₂. It has been analytically demonstrated that in case of an electron wave falling along the first (lower) quantum-dimensional subband in QW₁ and its kinetic energy E_x being less than the energy positions of all the other subbands in QW₁ (i.e., the undamped propagation of the wave reflected from the barrier with real quasi-momentum is possible only along this lower subband) еj_x⁽¹⁾(x,z) and еj_x⁽²⁾(x,z) are equal to zero. However, if a particle has such an energy that the refection of the wave with real quasi-momenta is possible along more than one (lower) subband, then the situation completely changes due to the interference of the reflected waves. In this case the interference leads to an existence of a complicatedly oscillating spatially inhomogeneous distribution еj_x⁽¹⁾(x,z), and under the barrier in QW₂ it provides the appearance of exponentially damped at х→∞ and possessing a coordinate dependence of leakage еj_x⁽²⁾(x,z) under the barrier. Besides, three regions of the symmetric along z axis propagation еj_x⁽²⁾(x,z) are formed under the barrier. They are the central one, in which the current is directed in axis x positive direction, and two side regions in which the current is directed in negative direction. The presence of the regions of that kind provides the charge flow from under the barrier. The numerical calculations of еj_x⁽¹⁾(x,z) and еj_x⁽²⁾(x,z) have also been made taking into account 31 subbands. It should be noted that these kinds of effects have a general nature and exist in 1D and 2D nanostructures with arbitrary profiles of QWs and barriers.

A smooth, ultra-flexible, and transparent electrode was developed from Ag nanowires (AgNWs) embedded in a colorless polyimide (cPI) by utilizing an inverted film-processing method. The resulting AgNW-cPI composite electrode is highly transparent and has an ultra-smooth surface with a low sheet resistance. The thickness of this conductive composite film was reduced to less than 100um with extreme flexibility. This film exhibited mechanical durability up to a bending radius of 5 mm. Green quantum dots light-emitting diodes (QLEDs) were fabricated using these composites as bottom electrodes (anodes). Hole-injection in QLEDs was poor, because AgNWs were largely buried beneath the composite’s surface. Thus, we used a simple plasma treatment to remove the thin cPI layer overlaying the nanowires without introducing other conductive materials. As a result, we were able to finely control the flexible QLEDs electrical/optical properties using the enlarged conductive pathways. The fabricated flexible devices showed only slight performance degradation after a repeated bending test.

The ultimate aim of molecular electronics is to overcome the limit of the conventional silicon based solid-state electronics by utilizing either single molecules or a bundle of molecules as an active electronic device component. For example, Nijhuis et al. recently reported a robust molecular diode using ferrocene-alkanethiolate self-assembled monolayers (SAMs) and eutectic Ga and In (EGaIn) electrodes.[1] They demonstrated a large rectification ratio of up to ~1000 and reported that the strong asymmetric electrical characteristics could be interpreted by hopping assisted tunneling transport arising from the ferrocene-alkanethiolates and the electrodes.

Similar to these studies, we have also examined the possibility of the electronic device application by fabricating a large number of molecular devices based on ferrocene-alkanethiolate (denoted as FcC) SAMs using a conventional solid-state device fabrication technique both on rigid and flexible substrates.[2] Specifically, we observed a distinctive temperature dependence on the electrical characteristics; that is, the current density decreased as the temperature increased in a certain temperature range when a sufficient voltage was applied with a certain voltage polarity. This behavior was in contrast to the usually expected thermally activated charge transport in the molecular device junction or other device junctions in which, most times, the current density increases as the temperature increases. In that study,[2] we suggested the unusual thermal characteristics are probably due to the redox-induced conformational changes of the FcC in the molecular junctions. While the analysis was quite reasonable and consistent with the experimental results, further evidence was needed to support our suggested explanation. We also performed temperature-dependent transition voltage spectroscopy (TVS) analysis based on a multibarrier tunneling model, which supports the occurrence of the proposed redox-induced conformational changes in the FcC molecular junctions.[3]

References

1. L. Yuan et al., Nano Lett. 15, 5506 (2015).

2. H. Jeong et al., Adv. Funct. Mater. 24, 2472 (2014).

3. H. Jeong et al., J. Phys. Chem. C 120, 3564 (2016).

In this work, we have investigated the kinetic process and adatoms mechanism of four different top shapes of catalyst-free Ga(In)N nanorods (NRs) namely flat, taper, hammer, and mushroom structures were grown in holes of a patterned Si (111) substrate by plasma-assisted molecular beam epitaxy. Arrays of nano-holes with diameter of 80 nm on the Si substrate were obtained by using of self-assembled silica nanospheres as a nano-hole mask. The silica nanospheres coated on the Si substrate and then spread using a spin-coating method. A 10 nm-Ti deposited after dry etching to sizes control and then removed silica nanospheres by chemical etching. Different top shapes of Ga(In)N were obtained by varying the growth conditions namely growth temperature and N2 plasma power, and the morphology evolution was explained based on the interrelation between sidewall diffusion and direct impingement during the NRs growth. GaN NRs grown at the growth temperature (T_g) and Ga flux of 650 °C and 5x10^-7 Torr, respectively, were used as the buffer for the subsequent growth of GaN NRs with different top shapes. GaN NRs with flat top (Fig. 1(a)) was obtained without altering the growth conditions that was used for the buffer growth. When the T_g was increased gradually from 590 °C to 650 °C under Ga flux of 1x10^-7 Torr, we obtained GaN NRs with top tapered shape (Fig. 1(b)). On the other hand, hammer shaped GaN NRs (Fig. 1(c)) were obtained when the T_g was reduced gradually to 570 °C, while keeping the Ga flux 1x10^-7 Torr during the growth of GaN NRs. Furthermore, mushroom shaped Ga(In)N nanostructure (Fig. 1(d)) on top of GaN NRs were obtained when introducing In flux of 1x10^-7 Torr at T_g ~ 350 °C under the same Ga flux. The variations in the shape of GaN NRs are explained by the interrelation of sidewall diffusion and the supersaturation of adatoms at the top surface [1, 2]. Photoluminescence measurements revealed higher light emission for tapered GaN when compared to non-tapered structure. APSYS simulations were further conducted to theoretically confirm the observed experimental results. We believe that our results can provide crucial information for the shape controlled growth of GaN NRs with diverse nanostructures and promising approach for the realization of high brightness LEDs.

Fig. 1 SEM images of GaN NRs with different top shape (a) flat, (b) tapered, (c) hammer and of GaInN with (d) mushroom

REFERENCES

[1] Xin Yin, Jian Shi, Xiaobin Niu, Hanchen Huang, and Xudong Wang, Nano let., 15, 7766 (2015)

[2] T. Stoica, R. Meijers, R. Calarco, T. Richter, and H. Luth, J. Cryst. Growth 290, 241 (2006)

Recently biosynthetic methods employing either biological entities or plant extracts have emerged as an easy, fast and economical alternative to chemical and physical síntesis procedures for the production of safer nanomaterials for human use. An eco-friendly semi-green method was used in order to obtain magnetite magnetic nanoparticles (Fe₃O₄-MNPs). Besides the know effect of polyphenols as therapeutical agents in cancer diseases, biomolecules from aqueous extracts can act as capping and reducing agents wich effectively replace toxic chemical reductans. Plant extracts with a rich mixture of active biological phytochemicals (i.e. polyphenol compounds, tannins, saponnins, flavonoids) control and shape the growing nanoparticles. Superparamagnetic properties have been studied extensively due to their potential use in hyperthermia in cancer treatment. The green synthesis of Fe₃O₄-MNPs with aqueous extracts represent a major advantage in the synthesis of superparamagnetic materials for biomedical usage, due to their diminished toxicity to biological organisms and more efficient drug delivery carriers for specific cancer diseases. In order to explore the diversity of biomolecules in the obtention of Fe₃O₄-MNPs, in this work an aqueous extract from Cinnamomun verum and Vanilla planifolia (natural pods and synthetic extract) were used during the synthesis of magnetite.

The Fe₃O₄ MNPs obtained were identified by XRD (PDF-19-0629) corresponding to an inverse spatial group Fd3m (227) inverse spinel FCC structure, a= 8.355 Å in synthetic vanilla and a=8.362 Å in vanilla pods extract (V. planifolia), and a=8.366 Å in C. verum. IR peaks at 576 cm^-1 correspond to Fe-O bonding formation; vibrational peaks at 576-1641 and 3415 cm^-1 suggest phenol molecules involved in bio-reduction process. XRD and HRTEM diffraction patterns overlap with the corresponding Fe₃O₄ peaks (220),(311),(400),(511),(440). The d spacing 2.4 Å in V. planifolia and 2.7 Å in C. verum match the main diffraction plane 35° (311). The particle size calculated by Scherrer´s equation by Scherrer´s equation (t=Kl/b cos Ɵ) in V. planifolia was 12 nm and 14 nm in C. verum. AFM-MFM data show a monodomain arrangement of 2-3 nm in V. planifolia and 5-6 nm in C. verum. VSM data indicate that magnetization increases rather using C. verum extract (64.89 emu/g) than V. planifolia (46.6 emu/g). The SPA values suggest that vanilla pods extract has an advantageous performance during Fe₃O4-MNPs synthesis due to their increased heating capability (64.51 W/g). The bio-synthesis of Fe₃O₄ MNPs obtained by aqueous plant extracts are commensurable to those obtained by a chemical method with a better performance than synthetic extract.

References.

[1] G. Canizal, P. Schabes-Retchkiman. Mater. Chem. Phys. 97 (2006) 321.

[2] R. Herrera, J.L. Rius, C. App. Phys. A 100 (2010) 453.

[3] G. Rosano-Ortega, P. Schabes-Retchkiman. American ScienIfic Publishers 6 (2006) 1.

[4]J.Chomoucka,J.Drbohlavova,D.Huska.PharmacologicalResearch62(2010)144.

[5] A. Ito, H. Honda, T. Kobayashi. Cancer Immunol Immunother 55 (2006) 320.

[6] N. Anton, J.P. Benoit, P. Saulnier. Journal of Controlled Release 128 (2008) 185.

[7] J. Santoyo-Salazar, L. Perez, O. de Abril. Chem Mat. 23 (2011) 1379.

[8] J. Santoyo-Salazar, M.A. Castellanos. J. Scanning Probe Microscopy 4 (2009) 17.

[9] Goya, G.F. ; Grazu, V.; Ibarra, M.R. Current Nanoscience 4(2008) 1.

We investigated a tungsten (310) surface, which has been widely used as electron emitting surface of cold field emitters (CFE). The intensity of electron beams emitted from a CFE rapidly decreases even under an ultra-high vacuum (UHV) environment of 10^-8 Pa [1]. There has been a recent report that a stable operation region with high current angular density appeared by operating a CFE under an extreme-high vacuum (XHV), from 10^-9 to 10^-10 Pa[2]. That study revealed that the decreasing of the emitted beam is caused by residual hydrogen gas in CFE gun chambers, and they used a non-evaporative getter (NEG) to evacuate the hydrogen gas from the gun chamber. In the stable operation region of a NEG-evacuated CFE, we have observed unexpected increasing and step-wise shifting of the beam current.

In order to understand the mechanisms behind the unexpected beam fluctuations, we performed electron emission experiments and computational simulation of the emission surface. Results showed that the step-wise fluctuation is related to the adsorption of a single carbon monoxide (CO) molecule onto the W(310) surface. We performed a first principle calculation of the CO-adsorbed W(310) surface in order to understand the mechanism behind the step-wise fluctuation. To study the site of electron emission on the single-CO-molecule-adsorbed surface, we developed a calculation method to evaluate the heterogeneous potential barrier height of the surface under strong electric fields. On the basis of these experimental and calculation results, we propose that the adsorption and diffusion of a single CO molecule on W(310) surface cause the step-wise fluctuations of the current.

[1] S. Yamamoto, et al., “Field emission current instability induced by migrating atoms on W(310) surface”, Surf. Sci., 71., 191 (1978)

[2] K. Kasuya, et al., “Stabilization of tungsten 310 cold field emitter”, J. Vac. Sci. Technol. B, 28, L55 (2010)