Nanofabrication with Pulsed Lasers
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An overview of pulsed laser-assisted methods for nanofabrication, which are currently developed in our Institute (LP3), is presented. The methods compass a variety of possibilities for material nanostructuring offered by laser–matter interactions and imply either the nanostructuring of the laser-illuminated surface itself, as in cases of direct laser ablation or laser plasma-assisted treatment of semiconductors to form light-absorbing and light-emitting nano-architectures, as well as periodic nanoarrays, or laser-assisted production of nanoclusters and their controlled growth in gaseous or liquid medium to form nanostructured films or colloidal nanoparticles. Nanomaterials synthesized by laser-assisted methods have a variety of unique properties, not reproducible by any other route, and are of importance for photovoltaics, optoelectronics, biological sensing, imaging and therapeutics.
KeywordsLaser nanofabrication Laser ablation Semiconductor and plasmonic nanostructures Colloidal nanoparticles
When nanostructured, many materials start to exhibit new optical properties making them unique for a plethora of applications. In particular, despite small and indirect band gaps in the bulk state, the nanostructured IV group semiconductors (e.g. Si, Ge) become efficient size-dependent emitters in the visible light range [1, 2], but also can work as photosensitizers to generate singlet oxygen under photoexcitation [3, 4]. Another prominent example relates to noble metal nanostructures, which provide a number of unique plasmonic effects, including size-dependent absorption peaks [5, 6], drastic local electric field enhancement [7, 8], resolution beyond the diffraction limit , nanotrapping  etc. These new properties of emerging nanomaterials appear to be extremely promising for photovoltaics and optoelectronics, as well as for biological sensing, imaging and therapeutics.
The employment of pulsed lasers offers a novel unique tool for nanofabrication . When focused on the surface of a solid target, pulsed laser radiation causes a variety of effects, including heating, melting and finally ablation of the target and such processes can lead to an efficient material nanostructuring, as shown in Fig. 1. First, the laser-assisted removal of material from the irradiated spot can result in a spontaneous formation of variety of periodic micro- and nanoarchitectures within this spot [12–16]. Second, laser ablation of material from a solid target leads to the production of nanoclusters [17–20]. When produced in gaseous environment or in vacuum, these nanoclusters can then be deposited on a substrate yielding to the formation of a nanostructured film [17, 21–24]. When produced in liquid environment, the nanoclusters can be released into the liquid forming a colloidal nanoparticle solution [25–32]. In all cases, properties of formed nanostructures can be unique and not reproducible by any other route [27–33]. As an example, the fabrication of nanoparticles in aqueous solutions does not require any chemical reducing agent, which conditions unique surface chemistry and purity of produced nanomaterials [28, 29]. Furthermore, when synthesized in clean, biocompatible environment, laser-synthesized nanomaterials are exempt of any residual toxicity that is typical for chemically synthesized nanoparticles [32, 33].
Laser-assisted Nanostructuring of Surfaces
Properties of laser-materials interaction are known to be strongly dependent on parameters of laser radiation. Among these parameters, the wavelength and pulse length are especially important to determine the efficiency of radiation absorption, dynamics of plasma plume expansion and nanoclustering . Ultraviolet, visible and near-infrared lasers are normally considered as most adequate for laser ablative tasks. Indeed, most industrially important materials efficiently absorb laser radiation in the spectral range of 200–1,000 nm, yielding to material ablation and nanostructuring. In addition, radiation of UV and visible lasers is relatively transparent to laser plasma, which minimizes laser beam distortions and power losses before reaching the target. In contrast, the interaction of infrared radiation (1–11 μm) with materials is characterized by a strong energy absorption by plasma itself, which completely changes conditions of nanostructuring. The pulse length is another important parameter in laser–matter interaction. Micro- and nanosecond laser–matter interactions are typically associated with long ablation regime, in which the ablation process takes place during the laser pulse itself. In contrast, pico- and femtosecond lasers offer short ablation regimes, in which moments of radiation absorption and material ablation are temporally separated. Although multi-pulse laser ablation from the target surface is always accompanied by the formation of micro- and nanoscale features and periodic structures [12–16, 34], properties of these features strongly depend on the wavelength and pulse length. Therefore, depending on the task, one can select appropriate radiation parameters to condition the ablation regime and obtain prescribed properties of laser-synthesized nanostructures. Below, we give several examples of laser-assisted methodologies to fabricate nanostructures within the irradiation spot.
Femtosecond Laser Ablation of Si: Formation of Black Si for Photovoltaics Applications
It is accepted that femtosecond pulses give at least two major advantages to micromachining compared to nanosecond and longer pulses [35, 36]: (1) the reduction of the pulse energy which is necessary to induce ablation for fixed laser wavelength and focussing conditions and (2) a significant reduction or complete removal of heat-affected zone (HAZ) and, as consequence, the improvement of the contour sharpness for the laser-processed structures. The second advantage is a direct consequence of the pulse being shorter than the heat diffusion time, given by the phonon transport. As in the case of nanosecond- and microsecond laser ablation, multi-pulse femtosecond ablation leads to a spontaneous formation of nanoarchitectures [14, 37–40]. However, in contrast to long ablation, the fs regime is characterized by the absence of target melting effects, yielding to the formation of clean micro- and nanoscale features. In particular, using multi-pulse fs ablation of Si in the presence of SF6 reactive gas, Mazur et al. [14, 37] managed to fabricate extremely narrow micro-spikes within the irradiation spot, which are capable of efficiently absorbing light in the visible and infrared ranges. The efficient absorption of light exceeding 95% in the visible was attributed to a geometric multi-reflection effect offered by a unique spike-based structure, while the enhanced absorption in the infrared was explained by a sulphur doping . Due to the wide-range absorption effect, the produced spike-based structure was called “black silicon” and was later used for the development of Si-based photodetectors. It is worth noting that such absorptive features cannot be reproduced by any alternative non-laser route. Other studies (see, e.g. ) reported the fabrication of nanostructured metal films exhibiting colours (“coloured metals”) using similar fs ablation approach.
Laser Plasma-assisted Nanostructuring of Surfaces
As we mentioned above, UV or ultrashort lasers contribute to a good radiation absorption by the target itself, while plasma remains relatively transparent to the incoming radiation. Such parameters ensure good quality of surface treatment in laser processing tasks. The plasma effect can be further minimized by reducing the pressure of the ambient gas. Depending on plasma plume size conditioned by the ambient gas pressure, the material can be re-deposited either within the irradiation spot (for high atmospheric pressure) or into the environment (for reduced pressures). In particular, for atmospheric pressure, the ablation process results in the formation of a deep crater, containing microscale spikes, covered by re-deposited nanoparticles [13–16]. In this case, chemical transformations in ablated species are minimal, since the ablated material rapidly cools down while interacting with the environment [24, 43].
Near-field Nanoparticle-assisted Nanostructuring of Surfaces: Fabrication of Patterned Nanoarrays
Laser Ablative Synthesis of Nanoclusters
The evolution of properties of formed nanostructures after the ablation process is mainly determined by the interaction of the species with the environment. In fact, in the first approximation, the nanocluster formation process can be described by the classical theory of condensation and nucleation in a vapour layer [64, 65]. However, the growth of nanoclusters strongly depends on initial parameters of ablated species (energy, angular distribution, density) and by laser interaction with the environment, which are conditioned by characteristics of pumping radiation. In particular, the radiation can directly affect the nucleation process if the pumping laser pulse is long enough. Indeed, the energy of photons can be sufficient to produce nucleation centres, change the dynamics of the nuclei growth and modify the diffusion of species in the vapour phase. Below, we give a review of experimental results on the laser ablative nanostructure growth.
Laser Ablation in Residual Gases and Deposition of Nanostructured Films
The first possibility is related to the ablation of material in gaseous environment and the deposition of ablated species onto a substrate in pulsed laser deposition (PLD) geometry. In this geometry, the employment of a UV radiation from an excimer laser is normally preferable . UV radiation is well absorbed by most industrially important materials, while the formed plasma plume is relatively transparent to it. The material can be in general ablated in vacuum; but due to a low probability of nanocluster coalescence in vacuum, it is difficult to control their growth and the nanoclusters normally deposit as ablated, forming a dense film with a significant amorphous fraction. Therefore, it is important to have a residual neutral light gas to affect the growth procedure. In this case, nanoclusters cool down under collisions with gas molecules or atoms, which contribute to their coalescence in the vapour phase. Under such conditions, the nanocluster growth process can be efficiently controlled by varying the pressure of the ambient gas [23, 66, 67].
To fabricate Si-based nanostructured films, we normally use radiation of a pulsed ArF or KrF lasers (193 or 248 nm, respectively, 15 ns FWHM, repetition rate 30 Hz) to ablate material from a rotating Si target. The radiation is focused at the incident angle of 45° to the surface. A substrate is placed on a rotating holder in front of the target. The experimental chamber is filled with helium for a deposition at a constant pressure ranging between 0.05 and 10 Torr. The film thickness after several thousands laser shots is 100–700 nm.
It is important that all Si-based films prepared by the PLD method exhibit strong visible emission (PL) under photoexcitation, while PL properties of the films are quite different for films of different porosities. Low-porous films P < 40%, deposited at reduced pressures P < 1.5 Torr, exhibit relatively weak PL with peak energy strongly depending on the gas pressure. In particular, the pressure decrease from 1.5 to 0.15 Torr in different depositions causes a blue shift of the peak from 1.6 to 2.15 eV, as shown in Fig. 6d. In contrast, films with an enhanced porosity P > 40%, deposited at higher pressures, provide only spectra with fixed peaks. The first band (1.6–1.7 eV) is independent of the pressure and can be seen just after the fabrication process. An additional 2.2–2.3 eV band can appear under the oxidation of samples in humid air. Here, PL properties of low-porous films (porosity <40%) are almost unchangeable under these conditions, while the integral PL intensity from highly porous films significantly increases with the prolonged oxidation. We attribute such a difference of PL properties to the impact of post-fabrication natural oxidation, controlled by the level of porosity. Dense and self-coagulated structures of the films fabricated under P < 1 Torr minimize the impact of ambient atmosphere on the film properties; and for these films, mechanisms related to core silicon crystals became predominant. Since the blue shift of the spectra under the decrease of helium residual pressure is accompanied by a certain decrease of the nanocrystal size, the quantum confinement mechanism  can be considered as one of main opportunities. In contrast, a high porosity enhances the surface area, which is subjected to surface chemistry modifications due to interactions of nanocrystallites with oxygen. This can drastically enhance the role of oxidation in the formation of PL centres and the domination of oxygen-related PL mechanisms connected either to defects in the SiO2 structure (usually, this mechanism provides 2–2.4 eV PL ) or to the interfacial layer (1.65 eV) .
Ultra-short Laser Ablation in Liquid Environment to Form Colloidal Nanoparticles
In the case of liquid ambience, laser ablation process leads to the release of nanoclusters into the liquid and the formation of a colloidal nanoparticle solution . In contrast to conventional chemical reduction methods, this method enables avoidance of the use of toxic chemical reduction agents to control the growth of the nano-particles. As an example, the laser ablation-based synthesis can be implemented in pure deionized water. The independence of laser-based synthesis of dirty colloidal chemistry makes it unique for the fabrication of markers of bioanalytes for sensing and imaging applications. In pure water, however, or any other aqueous solution exempted of additional chemically active components, the size of nanoparticles produced tends to be relatively large, since a natural coalescence of hot ablated nanoclusters cannot be easily overcome. In particular, nanosecond laser-based ablation used in most works, generally gives relatively large (10–300 nm) and strongly dispersed (50–300 nm) particles [25–28]. Although certain size control can be achieved by decreasing the wavelength of pumping radiation or varying the laser fluence, the range of size variations stays rather moderate in the nanosecond pulse case. Mafune  showed that size characteristics of nanoparticles can be improved by adding some reactive surfactants such as sodium dodecyl sulphate (SDS) or CTAB during the ablation. As an example, thiol-containing SDS covered gold nanoclusters just after their production and thus protected them from further coalescence. However, bioimaging applications of so produced nanoparticles are hardly possible since the surfactants are not biocompatible.
We recently proposed a femtosecond laser ablation-based method for nanoparticle synthesis, which makes possible an efficient control of size of prepared nanoparticles by varying physical parameters [28–33]. The experiments are normally carried out with an Ytterbium (400 fs FWHM, 1,025 nm, 1 kHz) or a Ti/Sapphire laser (110 fs FWHM, 800 nm, 1 μJ/pulse, 1 kHz). The radiation is focused onto a target of different materials (Au, Ag, Si, Ti, Cu), which is placed on the bottom of a glass vessel filled with aqueous solutions. The vessel is placed on a horizontal moving platform to avoid the ablation of material from the same area. The ablation experiments are carried out in pure deionized water and in aqueous solutions (biopolymers, cyclodextrins).
Another important issue is related to chemical properties of laser-synthesized nanoparticles. Since the nanoparticles are produced by pure physical ablation from a target without the involvement of any specific chemicals, surface chemistry of these nanoparticles can drastically differ from that of counterparts prepared by conventional colloidal chemistry [30, 31, 71–73]. In particular, laser-synthesized gold becomes susceptible to oxidation and, in contrast to chemically prepared gold, the surface of these nanoparticles is partially covered by a layer of oxide. Furthermore, the oxidized nanoparticle surface can have different termination, depending on the pH of the environment. The oxidized portion of the gold surface normally has Au–O– groups at pH > 5.8 and increasing numbers of Au–OH groups at pH < 5.8 . This oxide-related surface termination makes possible interactions of gold nanoparticles with groups, for which conventional gold is normally inert. A prominent example of new gold chemistry is related to the use of biopolymers  and oligosaccharides . Although these substances do not contain gold-reactive thiol group, they react with laser-synthesized gold nanoparticles, yielding to a drastic reduction of the nanoparticle size. Figure 7b illustrates the effect of the reduction of nanoparticle mean size under the use of a biopolymer polyethylene glycol (PEG). One can see that the nanoparticles’ mean size and size dispersion can be reduced down to 3 nm with the size dispersion not exceeding 1.5 nm FWHM. We believe that such reduction of nanoparticle size is a result of the hydrogen bonding of the –OH groups of these compounds and the –O− at the gold surface. The molecules of PEG cover gold nanoclusters just after ablation and act like “bumpers”, limiting contact between particles, preventing their coalescence (when the particles are still “hot”) and aggregation (when the particles are “cold”). Similar mechanism takes place during laser ablation in aqueous solutions of other polymers (dextran etc.)  and oligosaccharides (cyclodextrin) . Thus, OH groups of different biocompatible compounds can efficiently react with oxidized gold surface leading to the reduction of the nanoparticle size. It is important that in contrast to SDS or other surfactants previously used to control the nanoparticle size , biopolymers and oligosaccharides are essentially biocompatible. Moreover, the ultra-pure laser-ablated nanoparticles can be functionalized by a proper chemical modification of chemicals. We believe that this gives a huge advantage over the chemically produced nanoparticles for nano-engineering and functionalization of nanoparticles produced, as well as for a solution of toxicity problems. In particular, PEG is known as one of best materials to avoid the immune response in “in vivo” applications of inorganic nanoparticles. When covered by PEG, the nanoparticles become invisible for the immune system. In the case of laser-synthesized nanoparticles, one does not need to use any intermediate chemical group to link nanoparticles to PEG, as it takes place in the case of chemically synthesized nanoparticles.
We reviewed on the development, in our Institute, of various laser-assisted methodologies for nanofabrication in the gaseous and liquid environment. The methodologies imply the fabrication of nanoparticles/nanostructures either within the laser irradiation spot on the target surface or in the ablated species. Laser-synthesized nanomaterials exhibit unique optical properties and are exempt of toxicity, which make them very important for photovoltaics, optoelectronics, biological sensing, imaging and therapeutics.
The authors are grateful to Agence Nationale de Recherche (ANR) and Ion Beam Services (IBS company) for Plasma Immersion doping of the black silicon.
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