Yashar Mayamei | The University of Manchester (original) (raw)

Papers by Yashar Mayamei

physica status solidi (b), 2020

Graphene on hexagonal boron nitride (Gr/h-BN) exhibits interesting physics stemming from its Moir... more Graphene on hexagonal boron nitride (Gr/h-BN) exhibits interesting physics stemming from its Moiré superlattice, such as the fractional quantum Hall effect and superconductivity. [1-4] In research on this topic, devices based on graphene nanoribbons (GNRs) on h-BN (GNR/h-BN) with a 1D Moiré superlattice have been intensively studied for use in logic gates and spintronic devices. [5-9] In these systems, the confinement-gap energy of the GNRs, which enables a logic-gate operation, is inversely proportional to the width of the GNRs, W, [10] and is also affected by their chirality. [5] Gani et al. predicted that h-BN could induce very strong spin splitting at the edge states of a GNR on an h-BN substrate. [11] To realize oriented GNR/h-BN devices, Chen et al. used chemical vapor deposition to grow GNRs embedded in h-BN trenches created using a Ni-particleassisted etching method. [12] Epitaxial growth of GNRs along the atomic steps of the h-BN structure has also been demonstrated, where W was successfully controlled in the range 15-150 nm through modulation of the growth time. [13] Importantly, compared with the electron-hole charge fluctuations due to charge puddles in Gr/ SiO 2 , those in Gr/h-BN are suppressed by as much as two orders of magnitude, [14] allowing the development of a high-mobility Gr/h-BN device. [15] Given these properties of Gr/h-BN, a GNR/h-BN device is expected to show enhanced mobility compared with the corresponding GNR/SiO 2 device. [13] The origin of charge puddles has been mainly studied in Gr/SiO 2 systems. Using a scanning single-electron transistor, [16] Martin et al. showed that the charge density fluctuation due to charge puddles does not originate from trapped oxide charges in SiO 2 .I n another work, Zhang et al. showed that charge puddles with an average length scale of 20 nm result from charge-donating impurities between Gr and SiO 2 , not from topographical corrugations related to the surface of a substrate. [17] They suggested that, because the samples were prepared in ambient conditions, air molecules such as N 2 and H 2 O trapped between graphene and the SiO 2 substrate may be the origin of the charge puddles. Given this, it is necessary to know if the puddle landscape in a

Nanoscale, 2018

We report the first study of adsorption and evaporation of argon on conical alumina nanopores wit... more We report the first study of adsorption and evaporation of argon on conical alumina nanopores with different pore closures.

Nature Communications, 2018

Exposed to ionizing radiation, nanomaterials often undergo unusual transformations compared to th... more Exposed to ionizing radiation, nanomaterials often undergo unusual transformations compared to their bulk form. However, atomic-level mechanisms of such transformations are largely unknown. This work visualizes and quantifies nanopore shrinkage in nanoporous alumina subjected to low-energy ion beams in a helium ion microscope. Mass transport in porous alumina is thus simultaneously induced and imaged with nanoscale precision, thereby relating nanoscale interactions to mesoscopic deformations. The interplay between chemical bonds, disorders, and ionization-induced transformations is analyzed. It is found that irradiation-induced diffusion is responsible for mass transport and that the ionization affects mobility of diffusive entities. The extraordinary room temperature superplasticity of the normally brittle alumina is discovered. These findings enable the effective manipulation of chemical bonds and structural order by nanoscale ion-matter interactions to produce mesoscopic structures with nanometer precision, such as ultra-high density arrays of sub-10nm pores with or without the accompanying controlled plastic deformations.

Applied Physics A, 2016

In the present work, hard anodization assisted by anode temperature ramp was employed to fabricat... more In the present work, hard anodization assisted by anode temperature ramp was employed to fabricate selfordered nanoporous alumina in the wide range of interpore distances (259-405 nm) in pure oxalic acid and mixture of oxalic and phosphoric acid solutions. Anode temperature ramp technique was employed to adjust the anodization current density to optimize the self-ordering of the nanopore arrays in the interpore range in which no ordered selfassembled hard anodized anodic aluminum oxide has reported. It is found that the certain ratios of oxalic and phosphoric acid solutions in this anodization technique increased self-ordering of the nanopores especially for anodization voltages over the 170 V by increasing alumina's viscous flow which could lead to decrease the overall current density of anodization, yet leveled up by anode temperature ramp. However, below 150 V anodization voltage, the ratio of interpore distance to the anodization voltage of the both anodization techniques was the same (*2 nm/V), while above this voltage, it increased to about 2.2 nm/V.

Materials Chemistry and Physics, 2012

In aluminum anodization process the spontaneous current oscillation in certain electrolytes and a... more In aluminum anodization process the spontaneous current oscillation in certain electrolytes and anodization voltages can occur. The behavior of current in this process is called spontaneous oscillatory current otherwise we call the current behavior as non-oscillatory. The current difference between the spontaneous oscillatory and non-oscillatory conditions is appreciable (more than 50 mA/cm 2) whereas to switch from spontaneous oscillatory to non-oscillatory behavior we only need to change the anodization voltage less than 3 V. The pore structure of porous anodic alumina film can be modulated by this oscillatory behavior. This effect occurs due to the variation of the anodization current which causes the variation of pore diameter along the pores. But by this procedure it is hard to control the structure of the pore as it is required, because the modulated structure mainly depends on the spontaneous current oscillation. In this article, it is shown that this spontaneous oscillatory behavior can be switched to nonoscillatory condition by changing the anodization voltage. Therefore by switching the anodization voltage between the spontaneous oscillatory and non-oscillatory behaviors, in any time interval, the pore structure of nanopore alumina can be engineered.

Japanese Journal of Applied Physics, 2010

Well-ordered nanoporous arrays have been obtained using hard anodization of aluminium in oxalic/s... more Well-ordered nanoporous arrays have been obtained using hard anodization of aluminium in oxalic/sulfuric mixture. Various ordered nanoporous alumina films with pore intervals from 69 to 115 nm were fabricated on aluminum by high current anodization approach with various sulfuric concentrations in the oxalic/sulfuric mixture electrolyte under 36-60 V. The sulfuric acid concentration was changed from 0.06 to 0.2 M. Different

Langmuir : the ACS journal of surfaces and colloids, Jan 14, 2015

In the present work, hard anodization assisted by anode temperature ramp was employed to fabricat... more In the present work, hard anodization assisted by anode temperature ramp was employed to fabricate self-ordered nanoporous alumina in the wide range of interpore distances (259–405 nm) in pure oxalic acid and mixture of oxalic and phosphoric acid solutions. Anode temperature ramp technique was employed to adjust the anodization current density to optimize the self-ordering of the nano-pore arrays in the interpore range in which no ordered self-assembled hard anodized anodic aluminum oxide has reported. It is found that the certain ratios of oxalic and phosphoric acid solutions in this anodization technique increased self-ordering of the nanopores especially for anodization voltages over the 170 V by increasing alu-mina's viscous flow which could lead to decrease the overall current density of anodization, yet leveled up by anode temperature ramp. However, below 150 V anodization voltage, the ratio of interpore distance to the anodization voltage of the both anodization techniques was the same (*2 nm/V), while above this voltage, it increased to about 2.2 nm/V.

In aluminum anodization process the spontaneous current oscillation in certain electrolytes and a... more In aluminum anodization process the spontaneous current oscillation in certain electrolytes and anodization voltages can occur. The behavior of current in this process is called spontaneous oscillatory current otherwise we call the current behavior as non-oscillatory. The current difference between the spontaneous oscillatory and non-oscillatory conditions is appreciable (more than 50 mA/cm 2 ) whereas to switch from spontaneous oscillatory to non-oscillatory behavior we only need to change the anodization voltage less than 3 V. The pore structure of porous anodic alumina film can be modulated by this oscillatory behavior. This effect occurs due to the variation of the anodization current which causes the variation of pore diameter along the pores. But by this procedure it is hard to control the structure of the pore as it is required, because the modulated structure mainly depends on the spontaneous current oscillation. In this article, it is shown that this spontaneous oscillatory behavior can be switched to nonoscillatory condition by changing the anodization voltage. Therefore by switching the anodization voltage between the spontaneous oscillatory and non-oscillatory behaviors, in any time interval, the pore structure of nanopore alumina can be engineered.

We have carried out systematic experiments and numerical simulations of the adsorption on porous ... more We have carried out systematic experiments and numerical simulations of the adsorption on porous anodic aluminum oxide (AAO) duplex layers presenting either an ordered or a disordered interconnecting interface between the large (cavity) and small (constriction) sections of the structured pores. Selective blocking of the pore openings resulted in three different pore topologies: open structured pores, funnel pores, and ink-bottle pores. In the case of the structured pores having an ordered interface, the adsorption isotherms present a rich phenomenology characterized by the presence of two steps in the condensation branch and the opening of one (two) hysteresis loops during evaporation for the ink-bottle (open and funnel) pores. The isotherms can be obtained by summing the isotherms measured on uniform pores having the dimensions of the constrictions or of the cavities. The numerical analysis of the three different pore topologies indicates that the shape of the junction between the two pore sections is only important for the adsorption branch. In particular, a conic junction which resembles that of the AAO pores represents the experimental isotherms for the open and funnel pores better, but the shape of the junction in the ink bottle pores does not matter. The isotherms for the duplex layers with a disordered interface display the same general features found for the ordered duplex layers. In both cases, the adsorption branches coincide and have two steps which are shifted to lower relative pressures compared to those for the ordered duplex. Furthermore, the desorption branches comprise hysteresis loops much wider than those of the ordered duplex layers. Overall, this study highlights the important role played by morphologies where there are interconnections between large and small pores.

physica status solidi (b), 2020

Nanoscale, 2018

Nature Communications, 2018

Applied Physics A, 2016

Materials Chemistry and Physics, 2012

Japanese Journal of Applied Physics, 2010

Langmuir : the ACS journal of surfaces and colloids, Jan 14, 2015

In the present work, hard anodization assisted by anode temperature ramp was employed to fabricat... more In the present work, hard anodization assisted by anode temperature ramp was employed to fabricate self-ordered nanoporous alumina in the wide range of interpore distances (259–405 nm) in pure oxalic acid and mixture of oxalic and phosphoric acid solutions. Anode temperature ramp technique was employed to adjust the anodization current density to optimize the self-ordering of the nano-pore arrays in the interpore range in which no ordered self-assembled hard anodized anodic aluminum oxide has reported. It is found that the certain ratios of oxalic and phosphoric acid solutions in this anodization technique increased self-ordering of the nanopores especially for anodization voltages over the 170 V by increasing alu-mina's viscous flow which could lead to decrease the overall current density of anodization, yet leveled up by anode temperature ramp. However, below 150 V anodization voltage, the ratio of interpore distance to the anodization voltage of the both anodization techniques was the same (*2 nm/V), while above this voltage, it increased to about 2.2 nm/V.

In aluminum anodization process the spontaneous current oscillation in certain electrolytes and a... more In aluminum anodization process the spontaneous current oscillation in certain electrolytes and anodization voltages can occur. The behavior of current in this process is called spontaneous oscillatory current otherwise we call the current behavior as non-oscillatory. The current difference between the spontaneous oscillatory and non-oscillatory conditions is appreciable (more than 50 mA/cm 2 ) whereas to switch from spontaneous oscillatory to non-oscillatory behavior we only need to change the anodization voltage less than 3 V. The pore structure of porous anodic alumina film can be modulated by this oscillatory behavior. This effect occurs due to the variation of the anodization current which causes the variation of pore diameter along the pores. But by this procedure it is hard to control the structure of the pore as it is required, because the modulated structure mainly depends on the spontaneous current oscillation. In this article, it is shown that this spontaneous oscillatory behavior can be switched to nonoscillatory condition by changing the anodization voltage. Therefore by switching the anodization voltage between the spontaneous oscillatory and non-oscillatory behaviors, in any time interval, the pore structure of nanopore alumina can be engineered.