S.K. Garg

Temporal evolution of nanoporous layer in off-normally ion irradiated GaSb

Room temperature irradiation of GaSb by 60 keV Ar+-ions at an oblique incidence of 60° leads to simultaneous formation of a nanoporous layer and undulations at the interface with the underlying substrate. Interestingly, with increasing ion fluence, a gradual embedding of the dense nanoporous layer takes place below ridge-like structures (up to the fluence of 1 × 1017 ions cm−2), which get extended to form a continuous layer (at fluences ≥4 × 1017 ions cm−2). Systematic compositional analyses reveal the co-existence of Ga2O3 and Sb2O3 in the surface layer. The results are discussed in terms of a competition between ion-induced defect accumulation and re-deposition of sputtered atoms on the surface.

60 keV Ar+-ion induced modification of microstructural, compositional, and vibrational properties of InSb

Room temperature irradiation of InSb(111) by 60 keV Ar+-ions at normal (0°) and oblique (60°) angles of incidence led to the formation of nanoporous structure in the high fluence regime of 1 × 1017 to 3 × 1018 ions cm−2. While a porous layer comprising of a network of interconnected nanofibers was generated by normal ion incidence, evolution of plate-like structures was observed for obliquely incident ions. Systematic studies of composition and structure using energy dispersive x-ray spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, Raman mapping, grazing incidence x-ray diffraction, and cross-sectional transmission electron microscopy revealed a high degree of oxidation of the ion-induced microstructures with the presence of In2O3 and Sb2O3 phases and presence of nanocrystallites within the nanoporous structures. The observed structural evolution was understood in terms of processes driven by ion-induced defect accumulation within InSb.

Facile synthesis of a superhydrophobic and colossal broadband antireflective nanoporous GaSb surface

This paper reports the facile synthesis of tunable hydrophobic and colossal antireflective nanoporous GaSb by alteration of its porosity. In particular, it is observed that the contact angle of a water droplet on the GaSb surface increases as the nanoporous structures undergo different stages of growth and finally exceeds 150°, indicating the transition to a superhydrophobic surface. The observed correlation between the contact angle and the surface morphology is qualitatively understood in light of the Cassie–Baxter model. It is found that with the temporal evolution of nanostructures, a decrease in the fraction of the solid surface wetted by the water droplet and a corresponding increase in the air–water interface fraction lead to the enhancement in hydrophobicity, where the chemistry of the porous surface also plays a role. The temporal evolution of the contact angle is also studied to understand the interaction of the sessile drop with the hydrophobic surface and the ambient. In addition to an increase in the contact angle, we also observe a colossal broadband antireflection (in the range of 200–800 nm), which is correlated to a large reduction in the refractive index due to increasing porosity. Such a surface, with the combination of superhydrophobicity and colossal antireflection, can be very useful in the applications of GaSb nanostructures in thermophotovoltaic cells or photodiodes.

Tunable wettability of Si through surface energy engineering by nanopatterning

We synthesize nanoscale ripple patterns on Si surfaces by ion sputtering and show a systematic variation in the wettability of the surface depending upon its structure and chemical composition. As a matter of fact, our experiments reveal a hydrophilic to hydrophobic transition of the Si surface as a function of morphology and composition. Observed variation in the contact angle is found to be consistent with Wenzels law up to the onset of a sinusoidal ripple pattern formation on the Si surface, although a clear deviation from Wenzel-like behavior is detected after ripple formation. This deviation is attributed to a reduction in the surface free energy of ion implanted Si surfaces due to the formation of ripples. Further, we detect the existence of a top amorphous layer and the presence of Ar atoms near the top surface for all ion implanted Si samples. Thus, to understand our results, we take into account the compositional modification of the implanted surface due to incorporation of Ar atoms and attempt to correlate the observed evolution of contact angle with this compositional heterogeneity following the ripple formation. We have shown that the pinning behavior of a water droplet on the rippled-Si surface, due to the presence of ripple height, can be altered through a change in the surface composition.

Statistical analysis of ripple morphology on Si surfaces due to 60 keV Ar + -ions

We report on analysis of ion-beam patterned surface morphology in terms of regularity of pattern shape and orientation, homogeneity over irradiated surface, and the effective increment in its surface area, which are critical in deciding the applications for the corresponding surface. As a case study, we have chosen Si surface, which is exposed to 60 keV Ar+-ions at different angles of incidence and ion fluence and have performed detail statistical analysis of topographic images of the patterned surfaces. By using the Scanning Probe Image Processor (SPIP) software, morphological parameters, viz. surface area ratio, texture direction index, texture aspect ratio, ratio of system correlation length to ripple wavelength, directional roughness exponents, and anisotropy ratio are calculated as functions of ion incidence angle and fluence. From angle-dependent studies, we observe that ripple patterns become more regular with increasing angle of incidence. On the other hand, fluence-dependent study of these parameters shows that ripple shapes are most regular for the fluence of 3 × 1018 ions cm−2, while ripples are most unidirectional for the fluence of 2 × 1018 ions cm−2. Our analysis method shows a route towards optimization of ion-patterned surfaces in terms of nanostructure quality or effective surface area, which is vital for applications. Further, using scaling analysis, we associate Si surfaces generated within particular angular or fluence range to different universality classes, which can help towards understanding of their formation mechanism.

Evolution Of Surface Topography On GaAs(100) And GaAs(111) At Normal And Oblique Incidence Of Ar+‐Ions

Nanoscale surface structures emerging from medium energy (50–60 keV) Ar+‐ion sputtering of p‐type GaAs(100) and semi‐insulating GaAs(111) substrates have been investigated. For normally incident 50 keV Ar+‐ions of fluence 1×1017 ions/cm2 on GaAs(100) and GaAs(111) features in the form of nanoscale pits/holes without short range ordering are observed with densities 5.2×109 /cm2 and 5.9×109 /cm2, respectively along with irregularly shaped patches of islands. For GaAs(111) on increasing the influence to 5×1017 /cm2 the pit density increases marginally to 6.2×109 /cm2. For 60° off‐normal incidence of 60 keV Ar.+‐ions of fluence 2×1017 ions/cm2 on GaAs(100) microscale wavelike surface topography is observed. In all cases well‐defined nanodots are absent on the surface.

Facile synthesis of KCl:Sm3+ nanophosphor as a new OSL dosimetric material achieved through charge transfer between the defect states

Since precise control of nanoscale features is in high demand, it is being exploited to develop and improve OSL dosimetric materials, where striking improvement might also be expected in lanthanide-doped metal halides. The major challenge in the development of a nanophosphor lies in avoiding the aggregation of a dopant element in host materials, which has long prevented an in-depth exploration for the same purpose. This study focuses on the synthesis and characterization of Sm-doped KCl nanophosphors to develop a novel accession to investigate the highly sensitive trivalent Sm-doped KCl phosphor. Herein, we were able to overcome the aggregation phenomena and we showed that Sm-doped KCl with 0.45 mol% of Sm, which is the optimised dopant concentration, exhibits the high-intensity luminescence performance under blue light stimulation for the gamma doses in the range from 100 mGy to 1000 Gy. This sensitivity is attributed to the uniform nanospheres encapsulated in KCl along with the predominant existence of a trivalent (Sm3+) state, where these conditions can introduce additional defects centres. The presence of these additional defect centres was confirmed by photoluminescence studies, plausibly supporting the charge transfer due to the optical energy between these states, leading to high sensitivity. To establish KCl:Sm as a good OSL dosimetric materials (DM), we investigated fading, reusability, and reproducibility and compared these with those of commercial DM compounds such as Al2O3:C and BeO. Overall, Sm-doped KCl is non-toxic, cost-effective, robust, and a promising candidate for reusable dosimetry.

Anomalous behavior in temporal evolution of ripple wavelength under medium energy Ar+-ion bombardment on Si: A case of initial wavelength selection

We have studied the early stage dynamics of ripple patterns on Si surfaces, in the fluence range of 1–3 × 1018 ions cm−2, as induced by medium energy Ar+-ion irradiation at room temperature. Under our experimental conditions, the ripple evolution is found to be in the linear regime, while a clear decreasing trend in the ripple wavelength is observed up to a certain time (fluence). Numerical simulations of a continuum model of ion-sputtered surfaces suggest that this anomalous behavior is due to the relaxation of the surface features of the experimental pristine surface during the initial stage of pattern formation. The observation of this hitherto unobserved behavior of the ripple wavelength seems to have been enabled by the use of medium energy ions, where the ripple wavelengths are found to be order(s) of magnitude larger than those at lower ion energies.

Ion erosion induced nanostructured semiconductor surfaces

We consider nanostructures formed on semiconductor surfaces of Si(100), InP(100) and GaAs using medium energy (50–100 keV) Ar+–ion beam sputtering. The issues of dependence of nanostructure formation on these semiconductor substrates on ion–energy, –fluence, –flux, angle of incidence, and crystallographic orientation are addressed. The threshold fluence for formation of nano–islands on Si(100) implanted with normally incident 50 keV Ar+–ions was found to be 2 × 1017 ions/cm². For InP(100) implanted with 100 keV Ar+–ions an increase in angle of incidence results in decrease of surface roughness. Surfaces of GaAs(100) and GaAs(111) implanted with normally incident 50 keV Ar+–ions show nanopits of density 3–4 × 109/cm². The existing theories are applied to explain the formation of observed surface nanostructures.