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

Abstract

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.

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1. Introduction

Superhydrophobic surfaces are necessary for many applications, from self-cleaning windows to designing water repellant surfaces to manipulate fluid flow in microchannels for lab-on-chip devices, etc.¹ Micro electromechanical systems (MEMS) also require surfaces with low adhesion for proper operation.² On the other hand, development of antireflecting (AR) coatings with superhydrophobic properties is of prime importance for the improved operation of solar cells, LEDs, lenses, etc. since AR properties are necessary for light harvesting and superhydrophobicity leads to a superior device performance upon exposure to the environment due to the self-cleaning properties of the surface.³ Due to such requirements for the emerging applications, wide ranges of techniques have been utilized towards the synthesis of superhydrophobic and antireflecting surfaces.^4–11 Primarily, a surface is considered to be a superhydrophobic one if the contact angle, θ, becomes greater than 150°. In addition, a low contact angle hysteresis (difference between the advancing and receding contact angle), typically below 5–10° is required for self-cleaning properties, since this determines the roll-off of a water droplet on the surface.^12,13 The contact angle essentially depends upon the surface composition and the surface morphology. Examples of superhydrophobic and AR surfaces are abundant in nature. For instance, a lotus leaf demonstrates superhydrophobicity which arises due to its micro- and nano-hierarchical structures.⁶ AR nature of a moth eye over a broad range of wavelength is correlated to the hexagonal array of nanostructures.⁷ Mimicking such naturally occurring surfaces, many superhydrophobic^4,5 and AR^6,7 surfaces are prepared for different materials which comprise of different types of nanoscale surface structures.

However, designing-multifunctional surfaces combining the superhydrophobicity and antireflection has been rather challenging since optimization of the surface textures is essentially required to combine these two properties. Expectedly, preparation of the superhydrophobic and AR surfaces involves multiple steps.^7,14–16 Thus, a constant search is on for a large area, single step synthesis route for such materials surfaces.^3,7 For a III–V semiconductor, like GaSb, such a combination of the superhydrophobicity and AR nature of the surface is specifically important because it is considered to be a material of choice for thermophotovoltaic cells.^17,18 In addition, GaSb shows a better absorption compared to other semiconductors over a broad range of wavelengths, leading to the potential use of GaSb nanostructures for solar cell applications.^19,20 In fact, the peak of the solar spectrum is found to be at 550 nm and thus, the AR layer having a minimum reflection at this wavelength can be used to improve the efficiency of a solar cell. Different strategies of texturing is undertaken to reduce the reflection loss of GaSb,^17–19 whereas superhydrophobicity is preferred for the operation of the cells exposed to the environment.

In relation to the fabrication of surface nano and/or microstructures, ion implantation is a well-known and versatile method for a single step synthesis of novel materials over a large area and often in a self-organized manner.²¹ Recently, we have shown that nanoporous structures evolve in GaSb due to medium energy ion implantation.^22,23 These structures contain a network of interconnected nanofibers, whereas ridge-like structures develop on the top surface with increasing fluence.²³ In the present study, we show that the superhydrophobic and colossal broadband AR properties of nanoporous GaSb surface layer evolve with irradiation and are thus, tunable through changing the applied ion fluence. Our results indicate that ion implantation can be a method of choice for broad interests on a single step synthesis of multifunctional semiconductor surfaces and for other materials where porosity develops under ion irradiation.

2. Experimental section

The nanoporous GaSb structures are synthesized at room temperature (RT) by normally incident 60 keV Ar⁺-ion irradiation on mirror polished p-GaSb(100) wafers (sliced into areas of 1 × 1 cm²). The ion fluence was varied in the range of 7 × 10¹⁶ to 7 × 10¹⁷ cm⁻² with a constant current density of 10 μA cm⁻² in a high vacuum chamber with a base pressure of 8 × 10⁻⁷ mbar.²³ Microstructural evolution was investigated by scanning electron microscopy (SEM) in plan-view and cross-sectional modes (Carl Zeiss, Σigma), while the elemental analysis was carried out by the attached X-ray energy dispersive spectroscopy (EDS) system. Microstructural analysis of selective samples was carried out by transmission electron microscopy (TEM) (FEI, Tecnai G² F30), operating at 300 kV. In addition, grazing-incidence X-ray diffraction (GIXRD) measurements were carried out at an incidence angle of 5° (Bruker, D8-Discover) using the Cu-K_α radiation source (λ = 0.154 nm) to identify the phase and the nature of crystallinity in the porous structures formed in the near-surface region of GaSb. For compositional analysis, X-ray photoelectron spectroscopy (XPS) (VG Instruments) was performed on selected samples using the Mg-Kα radiation source (hν = 1254 eV). Contact angles of the water droplet on nanoporous GaSb surfaces were measured by the sessile drop method using a water droplet of volume 0.5 μL in a setup (Dataphysics, OCA15EC) having an accuracy of 0.1°. In order to check the uniformity of the surfaces, the contact angles were measured from a large number of randomly chosen places on the samples prepared at all fluences. Surface reflectance was measured using an ultra-violet-visible-near infrared (UV-Visible-NIR) spectrophotometer (Shimazdzu 3101PC), in the wavelength range of 300–800 nm with an unpolarized light. A specular geometry was used for these measurements where the incident light fell on the target at an angle of 45° with respect to the surface normal.

3. Results and discussion

The microstructural evolution of ion implanted GaSb samples is depicted by the SEM images in Fig. 1(a)–(d), corresponding to fluences in the range of 7 × 10¹⁶ to 7 × 10¹⁷ ions per cm², where a gradual but significant change in the surface morphology is evident with respect to the nominally flat (roughness: 0.9 nm) pristine sample (image not shown). Fig. 1(a) shows the development of terrace-like structures at the lowest fluence where an underneath network of nanofibers is clearly visible through the openings in between the same. The widths of the fibers are measured to be in the range of 15–25 nm. The underlying nanofibers get gradually exposed to incident ions [Fig. 1(b)] as the applied ion fluence increases to 1 × 10¹⁷ ions per cm². Randomly oriented ridge-like structures are formed over the exposed nanofibrous layer [Fig. 1(c)] at the intermediate fluence of 4 × 10¹⁷ ions per cm². On the other hand, at the highest fluence of 7 × 10¹⁷ ions per cm², the ridge-like structures are seen to have transformed to extended patches over the nanofibrous layer [Fig. 1(d)]. In fact, width of the ridge-like structures, in Fig. 1(c), is observed to be below 200 nm, whereas patches in Fig. 1(d) are more than 400 nm wide. Interestingly, widths of the nanofibers are observed to remain in the same range of 15–25 nm throughout the fluence range used in the present study.

Fig. 1 (a)–(d) Plan-view SEM images of the nanoporous structures evolved in GaSb at different stages of Ar-ion implantation. The ion fluences are marked over the images. XSEM images in (e) and (f) show the cross-sectional microstructures of GaSb corresponding to the plan-view images shown in (a) and (d), respectively.

In order to further explore the evolution of the nanoporous layer during ion implantation, cross-sectional scanning electron microscopy (XSEM) is performed on selected samples. Microstructure of the ion-beam fabricated nanoporous layer at the lowest fluence is depicted in Fig. 1(e), which clearly shows the formation of a porous layer of thickness ∼ 1 μm. In particular, near the top, the layer comprises of a network of nanofibers, while the columnar structures are observed below this network which extend down to the interface of the porous layer and the crystalline bulk GaSb underneath. On the other hand, XSEM image of the sample corresponding to the highest fluence reveals a highly modulated top layer with the presence of large protrusions on the surface [Fig. 1(f)]. The protrusions are, in fact, the patches observed in the corresponding plan-view image, [Fig. 1(d)]. Thus, we can infer that the patches formed over the nanofibrous layer are very rough. A fully grown layer of interconnected nanofibers is observed underneath the protrusions. This implies that the columnar structures observed at the lowest fluence transform into the nanofibers at higher ion fluences.

The processes that lead to the formation of a nanoporous layer containing the nanofibers are described in detail elsewhere.^22,23 In brief, during the early stage of ion irradiation, the vacancies which are created due to energy loss, agglomerate into voids due to inefficient recombination of vacancies and interstitials in GaSb. With increasing fluence, the voids grow in size since the vacancies created by further ion implantation migrate towards the existing voids. Finally, the growing voids get interconnected with each other to result in a porous microstructure, where the residual GaSb are observed as nanofibers. At the same time, the redeposition of sputtered atoms on top of the nanoporous layer leads to the formation of ridge-like structures [Fig. 1(c)], which subsequently extend into wide patches at higher fluences [Fig. 1(d)].

The modification resulted in the elemental compositions of the implanted GaSb samples, as probed by EDS (in the plan-view mode), is depicted in Fig. 2. In addition to Ga and Sb, O is also detected in the nanoporous layers, formed at all fluences. As a matter of fact, it becomes quite high in the nanoporous structures, viz. in the range of 22–26% for the entire fluence range (compared to that of 5.8% in the pristine sample). This large enhancement in the O atomic fraction can be attributed to an effective increment in the surface area of the implanted layers due to the increasing porosity (discussed below), causing accordingly an higher amount of O adsorption due to their subsequent exposure to air.²³ As a matter of fact, XPS measurements performed on the implanted samples (data not shown) reveal the signature of Ga₂O₃ and Sb₂O₃ phases to be present at the surface of the nanoporous layers upon O adsorption.²³ However, we do not observe the presence of Ar atoms due to the fact that they escape through the porous network.

Fig. 2 Variation in atomic concentrations of Ga, Sb, and O with ion fluence, as determined from EDS analyses of the samples.

Fig. 3(a) shows a XTEM image of the porous microstructure, formed at the lowest fluence. This image clearly demonstrates the formation of a porous layer at this fluence where the continuous top layer is the terrace-like structure observed in Fig. 1(a). It may be noted that the difference in thickness of the porous layer, while comparing the XSEM and XTEM images can be attributed to the local fluctuation in thickness (due to a very localized nature of observations made in case of a XTEM study). A high-resolution TEM (HRTEM) image of the porous GaSb structure is shown in Fig. 3(b) where the presence of lattice fringes in the HRTEM image clearly depicts the presence of GaSb crystalline pockets among large number of voids [as seen in Fig. 3(a)]. The measured d-spacing of 0.35 nm, obtained from the lattice fringes seen in Fig. 3(b), matches well with that of (1 1 1) plane of the cubic GaSb structure. The presence of GaSb crystallites within the nanoporous structures, throughout the fluence range used in the present study, is also confirmed from the GIXRD studies. Two clear peaks appear in the GIXRD spectra of the implanted samples [Fig. 3(c)], at 2θ = 25.4° and 42°, corresponding to the (1 1 1) and (2 2 0) reflections, respectively (for the cubic GaSb structure).²⁴ In addition, another peak is observed at 2θ = 50°, for fluences beyond 1 × 10¹⁷ ions per cm², which corresponds to the (3 1 1) reflection of cubic GaSb.²⁴

Fig. 3 (a) XTEM image of a GaSb specimen implanted with the fluence of 7 × 10¹⁶ ions per cm², (b) HRTEM image taken from the marked region in (a) at a higher magnification, and (c) represents GIXRD spectra of the samples irradiated to the two extreme fluences, the inset shows spectrum from the pristine GaSb(100).

In view of such temporal evolution of microstructures in GaSb during implantation, we have undertaken the study of addressing their functionalities and thus, perform a fluence-dependent evolution of the contact angle of a water droplet on top of the nanofibrous structures described above [Fig. 1(a)–(d)]. Fig. 4 summarizes the results where the contact angle of the water droplet on the pristine GaSb surface observed to be 100°, showing the hydrophobic nature of the same. However, with the formation of a nanoporous structure (at the lowest fluence) [Fig. 1(a) and 3(a)], a substantial increment in the contact angle, to 144°, is observed. As the porous nanofibrous structure evolves with increasing fluence, the corresponding contact angle increases further. For instance, at the intermediate fluences, the contact angle becomes 144.6° and 148.6°, respectively. Finally, at the highest fluence, where patches are formed on the nanofibrous structures, the contact angle increases to 151°. This clearly demonstrates the superhydrophobic nature of the nanofibrous surface developed at the highest fluence. The experimentally measured contact angles at different fluences are also listed in Table 1.

Fig. 4 Contact angle of water droplets measured as a function of the applied ion fluence. The lines are guide to the eyes. The insets show the photographs of water droplets on GaSb surfaces implanted to two extreme fluences (indicated above them). Corresponding to the fluences indicated in the insets, two schematic diagrams of water droplets on the respective surfaces are shown below them.

Table 1 Contact angles and estimated values of porosity along with reduction in refractive index at different fluences

Fluence (ions per cm^2)	Wetted solid fraction (as per eqn (2))	Contact angle, θ (degree)	Reduction in refractive index, (nd − n), as per eqn (6)	Porosity (%), as per eqn (5)
Pristine	1	100	0	0
7 × 10^16	0.231	144	2.82	71
1 × 10^17	0.224	144.6	3.31	88.5
4 × 10^17	0.177	148.6	3.41	94.1
7 × 10^17	0.152	151	3.43	95

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A modification in the wettability of textured surfaces is, in general, understood either in terms of Wenzel state²⁵ or Cassie–Baxter state.²⁶ In the case of Wenzel state of a water droplet on a rough surface, it completely wets the rough surface, filling in the opening in between the structures of the surface. In this situation, the modified contact angle, θ_W, is obtained from the following relation:²³

cos θ_W = r cos θ₀, (1)

where r is the ratio of the wetted surface area to its flat projection and θ₀ is the contact angle corresponding to the flat surface of the material. According to Wenzel's relation, a hydrophilic surface becomes more hydrophilic with increasing surface roughness, whereas a hydrophobic surface will be further hydrophobic.²⁵ However, when air is trapped within the water droplet and the hierarchical structures of a rough surface, the modified contact angle, θ_CB, follows the Cassie–Baxter state:²⁶

cos θ_CB = −1 + ϕ_s(1 + cos θ₀), (2)

where ϕ_s is the fraction of the solid surface in contact with the liquid.

In the present study, the pristine GaSb surface is found to be hydrophobic in nature. In addition, the highly porous nature of the surface, evolved under ion implantation, inevitably results in trapping of air molecules within the nanofibrous structures. Thus, the contact angle of the water droplet in this architecture is controlled by wetting of the top ridge-like structures or patches observed at different stages of ion implantation and the air trapped within the nanofibrous structures – just below the ridge-like structures/patches. Thus, it should be classified as the case of Cassie–Baxter state of a water droplet on the nanoporous GaSb structures and an increase in the contact angle up to 151°, resulting in a transition of the surface to a superhydrophobic one, can be attributed to a partial wetting of the surface features. The situation is schematically shown as insets in Fig. 4 for the lowest and the highest fluences. In fact, we have determined the wetted solid fraction of the surface of the structures [according to eqn (2)], at different fluences and have listed in Table 1. Interestingly, these values show a gradual decrease in the wetted solid fraction of the surface with increasing ion fluence. The decrease in the wetted solid fraction from unity (corresponding to the pristine GaSb surface) to 0.231 at the lowest fluence is clearly due to the formation of terrace-like structures on top of the nanofibers (with openings in between), as is observed in Fig. 1(a). This is shown in the schematic diagram corresponding to this fluence in Fig. 4. With an increase in the ion fluence, the fraction of the terrace-like structures decreases (discussed above) and nanofibers below them get exposed, leading to an increase in the contact angle. The reason for a further decrease in the wetted solid fraction at the intermediate fluence of 4 × 10¹⁷ ions per cm² can be attributed to the formation of ridge-like structures. The water droplet mostly wets the ridges, whereas the nanofibrous structures (below them) seen between the ridges leave a larger fraction of air–water interface compared to the fluence of 1 × 10¹⁷ ions per cm², since the open areas of the exposed nanofibers are larger in case of ridges [as is evident from Fig. 1(b) and (c)]. At the fluence of 7 × 10¹⁷ ions per cm², widths of the patches are higher and the exposed nanofibrous areas are evidently lower compared to the case of 7 × 10¹⁶ ions per cm². However, we note that protrusions are observed in the XSEM image of the surface at this fluence [Fig. 1(f)]. Therefore, it can be inferred that in this case the wetted portions of the surface are the apexes of these protrusions, while the gaps between them remain air-filled, resulting in a very small wetted solid fraction, as is demonstrated in the schematic diagram (the corresponding inset in Fig. 4).

The hydrophobic nature of a nanoporous GaSb surface can get further enhanced due to the presence of hydrocarbons on the surface, which is observed for other materials as well.^27,28 For instance, C adsorption is common for GaSb surface exposed to the ambient,²⁹ which can be more for a nanoporous GaSb surface formed due to ion implantation. These absorbed carbon atoms can get transformed into hydrocarbon compounds by reacting with water molecules adsorbed on the surface.^27,28 Therefore, we compare the C 1s XPS spectra obtained from the pristine GaSb and the one implanted to the fluence of 7 × 10¹⁶ ions per cm² (Fig. 5). From Fig. 5, an enhancement in the C content of the GaSb sample, subsequent to implantation, is evident. Similar trend is observed at higher fluences as well (data not shown to maintain the clarity). This increase in the C content is inferred to play a role as well in enhancing the hydrophobicity of the nanoporous GaSb surface. It should further be noted that XPS studies (data not shown) also show the presence of Ga₂O₃ and Sb₂O₃ on the ion bombarded GaSb surfaces due to oxygen adsorption, which corroborates well with higher O content in EDX data (as discussed above).²³ Consequently, a modification in the surface free energy due to the presence of these species is also expected to influence the evolution of contact angles.

Fig. 5 C 1s XPS spectra corresponding to the pristine GaSb and the one implanted to the fluence of 7 × 10¹⁶ ions per cm².

Temporal evolution of a water droplet on a hydrophobic surface is attracting increasing interest^30–33 because the underlying mechanism is important in many applications such as inkjet printing,³⁴ DNA chip manufacturing,³⁵ or evaporation driven self-assembly.³⁶ Therefore, we have also investigated the temporal evolution of the contact angle of the sessile drop on the pristine and the nanoporous GaSb surfaces, along with that of the contact radius, r_b, and height, h, of the sessile drop [illustrated in Fig. 6(a)] and the results are shown in Fig. 6(b)–(d). In Fig. 6(b), the contact angle of the water droplet on the pristine GaSb surface reduces from 100° to 8°, over a period of 400 s. On the other hand, only a little decrease in the contact angle corresponding to the nanoporous GaSb surfaces (formed at different fluences) is observed over a time span of 1800 s. In fact, the change in the contact angles with time becomes almost negligible corresponding to the highest fluence (i.e. for the highest porosity). Similarly, a nominal decrease in the contact radius of the water droplet (resting on the nanoporous GaSb surfaces) is observed [Fig. 6(c)], although a little increase in r_b with time is noticed for the pristine surface. On the other hand, the height h of the water droplet on the pristine sample is seen to decrease drastically within 400 s [Fig. 6(d)], whereas a systematic decrease in the same takes place (at all fluences) over a time span of 1800 s. Interestingly, here also, the change in the droplet height becomes minimal in case of the highest fluence (having the highest porosity, as discussed below).

Fig. 6 (a) Schematic diagram of a typical sessile drop on GaSb, having a spherical cap geometry with radius R_s. The contact angle, θ, contact radius, r_b, and drop height, h, are indicated in the diagram, the temporal evolutions of θ, r_b, and h are shown in (b), (c), and (d), respectively. The corresponding fluences are indicated in the plots.

For a sessile drop on a surface, h, r_b, θ, and radius (R_s) of the droplet [Fig. 6(a)] are interrelated and given by:^28,37

h = R_s(1 − cos θ) and r_b = R_s sin θ. (3)

The rate of change in the volume of a water droplet, constrained by a solid plane boundary, is given by Fick's law:^28,37

where V is the droplet volume, D is the diffusion coefficient of the water droplet, Δc (=C_s − C_∞) is the difference between the concentrations of water vapor at the drop surface and at an infinite distance, and ρ_L is the density of water. The function f(θ) can be chosen to be of the form f(θ) = (1 − cos θ)/2.²⁸

The diffusion constant can be determined from eqn (3) and (4) using the experimentally observed temporal evolutions of θ, r_b, and height, h, according to the methodology described in the supporting information. In the present experiment, the value of D corresponding to the pristine GaSb surface is found to be 4.3 × 10⁻⁵ m² s⁻¹. Corresponding to the nanoporous GaSb surfaces, evolved under fluences of 7 × 10¹⁶, 4 × 10¹⁷, and 7 × 10¹⁷ ions per cm² [Fig. 1], the values of D turn out to be 1.76 × 10⁻⁵, 1.12 × 10⁻⁵, and 1.06 × 10⁻⁵ m² s⁻¹, respectively. These values of diffusion coefficients of water are in reasonable agreement with the standard value of the diffusion coefficient of water vapor in air.³⁸ Consequently, the variation in the contact angles (corresponding to different fluences) with time can be attributed to evaporation of the water droplet in air.

Since it is well-known that hierarchical nanostructures are prone to a high degree of reflection loss,³⁹ we have performed the surface reflection studies on the nanoporous GaSb samples. The evolution of surface reflectance of light from the nanoporous GaSb surface, as a function of applied ion fluence, is depicted in Fig. 7. The surface reflectance of the pristine GaSb sample is also shown here for comparison, which shows that throughout the wavelength range of 300–800 nm, it remains above 50%. It should be mentioned here that the effect of the experimental geometry has been tested by measuring the surface reflectance after providing a perpendicular rotation (to all the samples) as well. However, within the experimental error, no difference in the reflectance values is detected under both the geometries. On the other hand, implantation to the lowest fluence brings the reflectance value down to 6.5% (at 550 nm). Subsequently, with increasing fluence, it undergoes a steady decrease and at the highest applied fluence, it dramatically drops down to ∼0.1%.

Fig. 7 Specular reflectance data of the porous GaSb samples as a function of the applied ion fluence. The surface reflectance data obtained from the pristine GaSb sample is also shown for comparison.

Such a colossal reduction in the surface reflectance can be correlated with the reduced refractive index, n, and a gradient in the same from top of the nanoporous layer towards the bottom.^40,41 As a matter of fact, the presence of air in between the nanoporous structures on the surface leads to a reduction in the refractive index of the material. To estimate the reduction in n due to the development of porosity, we have adopted the methodology of Kim, which is based upon Bruggeman's effective medium approximation:⁴²

where n is the refractive index of the porous layer and n_d is the refractive index of the pore-free pristine GaSb at 550 nm (i.e. 4.509).

Thus, the refractive index of the porous layer is given by:

where R is the surface reflectance of the layer at 550 nm.⁴³ The porosity and reduction in the refractive index, (n_d − n), obtained from eqn (5) and (6), are presented in Table 1 and also shown in Fig. 8, as a function of ion fluence. It is clear that the increment in porosity and the reduction in refractive index show similar trends. In other words, the evolution of porosity in the implanted GaSb samples leads to a reduction in the refractive index, and in turn causes a colossal reduction in the surface reflectance.

Fig. 8 Increase in the porosity (red filled circles) and reduction in the refractive index (n_d − n) (blue filled triangles) as a function of ion fluence. The lines are guide to the eyes.

Moreover, the microstructure of the nanoporous layer, as observed from the XSEM images in Fig. 1(e) and (f), indicates a variation in the reduced refractive index, n, from the top of the layer towards the bottom. This can be understood as follows: near the top of the irradiated samples, a much larger amount of air is expected to be trapped within the interconnected GaSb nanofibers, while the presence of nanocolumnar structures towards the bottom of the layer effectively leads to an enhanced amount of GaSb. In other words, the porous layer, evolving during implantation, has a gradually varying amount of GaSb from the top (∼100% air) to the bottom (∼100% GaSb). Thus, the refractive index gradually increases from the air/porous-layer interface on the top to the porous-layer/substrate interface at the bottom. It is this gradient in n, which is dramatically reduced due to the development of porosity, minimizing the reflection, leading to a colossal broadband AR from ion-implanted GaSb samples at room temperature.⁴⁰ The fact that the largest reduction in surface reflectance corresponds to the highest porosity corroborates well with the observation of minimal variation in the contact angle and droplet height at the highest fluence (discussed above). Further investigation in this direction in case of other implantation-induced nanoporous material is under way.

4. Conclusions

In summary, we have shown an effective route for fabrication of a superhydrophobic and colossal antireflective surface in a controlled manner through the gradual evolution of nanoporous structures by room temperature ion implantation in GaSb. It is observed that the evolved microstructures at different stages of ion irradiation result in a higher degree of hydrophobicity and antireflection before finally undergoing a transition to a superhydrophobic state at the highest fluence. We understand the observed superhydrophobicity in terms of the Cassie–Baxter state where air gets trapped between a water droplet and the nanofibrous structures, controlling the contact angle. The theoretically calculated wetted fraction of the surface area at different fluences, according to the Cassie–Baxter relation, is found to be well correlated with the observed ion-induced microstructural evolution. The modification in chemical composition of the nanoporous surfaces is also found to contribute in enhancing the hydrophobicity. Further, from the temporal evolutions of the contact angle, drop height, and drop radius (at different fluences), we have derived the diffusion coefficients and attributed the observed variations to the evaporation of water droplet with time. The antireflective nature of the nanoporous layers originates from a gradient in the refractive index, which reduces drastically due to the increasing porosity, arising out of a variation in the porous microstructure from the surface towards the depth. The present experimental findings will be extremely useful for application of GaSb in optoelectronic devices where antireflecting property as well as self cleaning properties are essential, such as thermophotovoltaic cells or solar cells.^17–19

Acknowledgements

Authors sincerely thank Mohit Kumar from SUNAG Laboratory, Institute of Physics, Bhubaneswar, and Sriparna Chatterjee, CSIR-IMMT, Bhubaneswar, for their helps during this study.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03778j

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