External field assisted laser ablation in liquid: an efficient strategy for nanocrystal synthesis and nanostructure assembly

External field assisted laser ablation in liquid an efficient strategy for nanocrystal synthesis and nanostructure assembly ( ) DOI http //dx doi org/10 1016/j pmatsci 2017 02 004 Reference JPMS 436 T[.]

( ) DOI: http://dx.doi.org/10.1016/j.pmatsci.2017.02.004

To appear in: Progress in Materials Science

Received Date: 27 January 2016

Revised Date: 20 February 2017

Accepted Date: 21 February 2017

Please cite this article as: Xiao, J., Liu, P., Wang, C.X., Yang, G.W., External field-assisted laser ablation in liquid:

an efficient strategy for nanocrystal synthesis and nanostructure assembly, Progress in Materials Science (2017), doi: http://dx.doi.org/10.1016/j.pmatsci.2017.02.004

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External field-assisted laser ablation in liquid: an efficient strategy

for nanocrystal synthesis and nanostructure assembly

J Xiao, P Liu, C X Wang, G W Yang*

State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P R China

*Corresponding author: stsygw@mail.sysu.edu.cn

Abstract

Laser ablation in liquid (LAL) has received considerable attention over the last decade, and is gradually becoming an irreplaceable technique to synthesize nanocrystals and fabricate functional nanostructures because it can offer effective solutions to some challenges in the field of nanotechnology The goal of this review is to offer a comprehensive summary of recent developments of LAL in nanocrystal synthesis and nanostructure fabrication First, we will introduce the fundamental processes of microsecond, nanosecond, and femtosecond LAL, and how the active species act differently in plasma, cavitation bubbles, and droplets in the different LAL processes Second, a variety of LAL-based techniques for nanomaterials synthesis and processing are presented, such as electric-, magnetic-, and temperature-field LAL, as well as electrochemically assisted LAL, pulsed laser deposition in liquid, and laser writing of nanopatterns in liquid Third, new progress in LAL-generated nanomaterials is described Fourth, we emphasize five applications of LAL-generated nanomaterials that have emerged recently in the fields of optics, magnetism, environment, energy, and biomedicine Finally, we consider the core advantages of LAL, the limitations of LAL and corresponding solutions, and the future directions in this promising research area

Contents

1 Introduction 7

2 Laser ablation of a solid target in liquid 13

2.1 Fundamental physical process 14

2.1.1 Millisecond-laser ablation in liquid and melting process 14

2.1.2 Nanosecond-laser ablation in liquid and plasma formation 16

2.1.3 Picosecond-laser ablation in liquid and cavitation bubble 21

2.1.4 Femtosecond-laser ablation in liquid and multiple processes 23

2.2 Fundamental chemical process 26

2.2.1 Chemical reactions in plasma 26

2.2.2 Chemical reactions at interface between solid and liquid 30

3 LAL-based nanomaterials preparation and processing 33

3.1 Electrical field-assisted LAL for shape controlling of metal oxide nanocrystals 33

3.1.1 Metal oxides semiconductor nanocrystals with various shapes 33

3.1.2 Influence of electrical field on shape formation of nanocrystals 34

3.2 Electrical field-assisted LAL for nanostructures fabrication 37

3.2.1 Functional nanostructures of metal oxides fabrication 37

3.2.2 Orientated attachment mechanism of nanostructures fabrication 38

3.3 Room temperature ripening-assisted LAL for nanostructures fabrication 39

3.3.1 One-and two-dimensional nanostructures fabrication 39

3.3.2 Ostwald ripening mechanism of nanostructures fabrication 40

3.4 Electrochemistry-assisted LAL for complex nanostructures fabrication 44

3.4.1 Fabricating simple polyoxometalate nanostructures 44

3.4.2 Chemical reactions in fabrication of nanostructures 46

3.5 Magnetic field-assisted LAL for magnetic nanochains fabrication 48

3.5.1 Fabrication of one-dimensional chain bundle of magnetic nanoparticles 48

3.5.2 Magnetic field induced orientated attachment mechanism 50

4 Nanopatterning in liquid 54

4.1 Pulsed-laser deposition in liquid for nanopattern fabrication 54

4.1.1 Pulsed-laser deposition in liquid 54

4.1.2 Fabrication of nanoparticles pattern on transparent substrates 54

4.2 Laser-writing of functional nanopatterns in liquid 55

4.2.1 Fabrication of nanopatterns with heterostructure 55

4.2.2 Phase transformation upon laser-writing in liquid 56

5 New progress in nanomaterials synthesis based on LAL 57

5.1 Nanoparticle–polymer composites 57

5.2 Doped semiconductor nanocrystals 60

5.3 Submicrometer spherical particles 63

5.4 Monodispersed colloid quantum dots 66

6 Applications in nanocrystals synthesis and nanostructure fabrication 68

6.1 Functional nanostructures for optics 69

6.1.1 Fluorescence emission 69

6.1.2 Visible light scattering 73

6.1.3 Nonlinear optics 75

6.2 Functional nanostructures for magnetics 75

6.3 Functional nanostructures for environmental applications 77

6.3.1 Adsorption 77

6.3.2 Photocatalytic degradation 79

6.3.3 Sensing 83

6.4 Functional nanostructures for green energy 87

6.4.1 Supercapacitor 87

6.4.2 Lithium-ion battery 88

6.4.3 Solar cell 89

6.4.4 Hybrid light-emitting diode 91

6.4.5 Water-splitting photocatalyst 92

6.4.6 Electrocatalyst 94

6.5 Functional nanostructures for biomedicine 96

6.5.1 Biomolecules carrier 96

6.5.2 Positive contrast agent 97

6.5.3 Bio-recognition 99

7 Conclusion and Perspective 100

7.1 Main advantages of LAL 100

7.1.1 Surface 100

7.1.2 Metastable structures 104

7.2 The main disadvantages of LAL and methods to address them 106

7.2.1 Productivity 106

7.2.2 Size and dispersity control 109

7.3 Future directions of LAL 111

7.3.1 Mechanisms exploration 111

7.3.2 Extending applications 113

Acknowledgments 118

References 119

1 Introduction

Although nanomaterials have been investigated extensively in recent decades, researchers still face fundamental challenges For example, how to control the phase, size, and shape of building blocks in nanomaterials synthesis, how to facilely fabricate functional nanostructures using these building blocks, and how to achieve the transformation from simple synthesis of nanomaterials to complex fabrication of functional nanounits [1–4] To address these issues, researchers have developed a series of conventional techniques to achieve a variety of functional objectives; for example, thermal chemical vapor transport based on the vapor–liquid–solid process for nanowires [5–8], solution-based chemical reactions for nanocrystals [9–11], and template assembly based on DNA molecules to obtain complex nanostructures [12–14] Among these methods, laser ablation in liquid (LAL) has drawn great attention in recent years because of its distinctive geometries [15–20] These include novel metastable nanophases and nanoparticle colloids with extremely high stability and purity LAL is a facile general technique with an almost unlimited variety of prospective materials and solvents [16,17]

Compared with conventional production processes of nanomaterials, such as gas-phase methods, which usually produce agglomerated micro- or nanopowders that are difficult to disperse to form functional matrices, and chemical methods, which generally provide nanomaterials with impurities originating from additives and precursor reaction products, LAL has the following advantages (i) LAL is a chemically simple and clean because the process has little byproduct formation,

simple starting materials, and no need for catalyst These factors ensure production of highly pure clean surfaces that often possess high surface activity [21] (ii) LAL is conducted under ambient conditions and does not require extreme temperature and/or pressure Despite the mild conditions, LAL still allows access to a variety of metastable phases that may not usually be attainable (iii) LAL is a facile general method with an almost unlimited scope of suitable materials and solvents Because new phase formation in LAL involves both a liquid and solid, researchers can choose and combine interesting solid targets and liquids to synthesize nanocrystals and fabricate nanostructures for fundamental research and potential applications [22] (iv)

In some cases, the phase, size, and shape of nanostructures can be readily controlled

by tuning laser parameters and assisting factors, allowing both nanocrystal synthesis and nanostructure fabrication in one-step For example, Koshizaki et al [23] used laser selective heating to synthesize submicron spheres of different size by changing the laser parameters Yang and colleagues used magnetic field-assisted laser ablation

in liquid (MFLAL) [23] to fabricate microfibers of an iron–carbon composite [24], submicron Co3C particle chains [25], and one-dimensional chains of iron-based bimetallic alloying nanoparticles [26]

Therefore, LAL has been proven a general and effective technique to synthesize nanomaterials and fabricate functional nanostructures Note that the achievements realized in this field in very recent years have greatly exceeded those made in the previous two decades Therefore, it is timely to review the recent progress of LAL applications in the preparation and processing of nanomaterials Next we briefly

summarize the recent developments in LAL

Fig 1a and b show the increasing number of articles and corresponding citations from 2000 to Nov 1st, 2016 in the field of LAL These data were obtained by searching the entry “Laser ablation in liquids” in the Web of Science Using different search string and data set refinement may provide different results [27] These two indexes steadily increase over the years, indicating the rapid development of LAL research Fig 2a and b present the advanced optical detection techniques used in LAL, such as optical emission spectroscopy for plasma characterization, the fast shadowgraph method to probe plasma and cavitation bubble dynamics, laser scattering to determine the delivery mechanisms of the produced materials in the liquids, and small-angle X-ray scattering to observe nanoparticle formation in a cavitation bubble [28,29] Additionally, researchers have developed various innovative types of equipment to achieve LAL under extreme conditions Fig 2c

depicts a photograph of a high-pressure cell designed for in situ experiments at

temperatures up to 400 K and pressures up to 30 MPa [30] All these experimental and detection devices greatly enrich the research on mechanism exploration and nanocrystal synthesis

The introduction of new laser, detection, and experimental devices makes LAL become a more powerful and efficient approach to fabricate new nanocrystals and explore synthetic mechanisms Because of the almost unlimited number of materials and various liquid solvents suitable for LAL, it has been used to synthesize a huge variety of advanced materials [23,31] Meanwhile, picosecond (ps) lasers that operate

at high power with a high repetition rate can markedly improve the yield of nanoparticles synthesized by LAL [32,33] Importantly, the progress in LAL is strongly stimulated during the biannual Advanced Nanoparticle Generation And Excitation by Lasers in Liquids (ANGEL) conference, which brings the international community together to discuss scientific issues in this field The first ANGEL conference was held in 2010 In just six years, this conference has become the most influential in the field of LAL The 2016 ANGEL conference covered almost every important advance in both theoretical and experimental aspects of LAL, such as spectroscopic, videography, and radiography studies [34–37], cavitation bubble dynamics [38,39], nanoparticle scale-up and methodological improvements [32,33], energy applications, and biomedical applications [40–42]

The laser source is recognized as the most important parameter in the synthesis

of nanoparticles by LAL One decade ago, LAL research mainly used nanosecond (ns) lasers, and nanoparticle synthesis also focused on this type of laser The fundamental understanding of other types of lasers with different pulse widths is limited compared with that of ns lasers To date, lasers with various widths have been used to perform LAL, including millisecond (ms) [43], microsecond (μs) [44], ns [45], ps [46], and femtosecond (fs) [47] lasers Researchers have proven that each type of laser has its unique advantages, which greatly widens the range of choices available to synthesize different types of nanomaterials as required Note that the interactions between materials in liquids and lasers with different pulse widths are quite different; for example, nanodroplets are created by a μs laser and a plasma plume is generated by a

to occur in LAL in extreme environments

External environment also greatly influences the LAL process Recently, various field-assisted LAL techniques have been developed, such as temperature field-assisted LAL [60,61], electric field-assisted laser ablation in liquid (EFLAL) [62,63], MFLAL [25,26], and electrochemistry-assisted laser ablation in liquid (ECLAL) [64,65] Great progress has been made following the introduction of field-assisted LAL Importantly, the experiments have proven that the morphology, composition, and structure of LAL-generated nanomaterials can be readily controlled by changing the external environment Additionally, many micro- and nanofabrication techniques based on LAL, like pulsed laser deposition (PLD) in liquid and laser writing of nanopatterns in liquids [66,67], have been developed These techniques have shown great potential in

the assembly of nanoparticles and nanopattern manufacture

By considering the above three factors, the morphology control and composition regulation of nanocrystals can be realized by LAL [68] For example, morphologies including nanorods [69], plates [70,71], layers [72], spindles [62,63], tubes [73], and hollow [43,74] and cubic particles [75,76] have been produced by LAL Various compositions have also been obtained through LAL, such as metals [77,78], oxides [79], nitrides [80], carbides [81,82], sulfides [83], selenides [84], alloys [64,65], and polyoxometalates (POMs) [65,69] Large numbers of new LAL-generated nanomaterials have been reported recently; some distinctive cases are summarized in Fig 3

The early stage in LAL research involved a handful of applications, such as controlling the size and dispersion of noble metal nanoparticles [85–87] Because of the accumulation of knowledge over almost two decades, this situation has changed markedly during the past few years LAL has gradually become a well-known alternative method to synthesize nanocrystals and fabricate nanostructures for practical applications in the optical, magnetic, and energy fields [88–90] Meanwhile, their distinct ligand-free active surfaces endow LAL-generated nanomaterials with great advantages in environmental and biomedical fields [54,91], as shown in Fig 4

In this review, we introduce the fundamental understanding of the LAL process and describe recent achievements of LAL in the synthesis of nanocrystals and nanostructure fabrication This review is organized into seven sections After this brief introduction of recent LAL developments in Section 1, we first explain the

fundamental understanding of LAL, i.e., the basic physical and chemical processes of LAL on the basis of the pulse width of the laser involved in nanocrystal synthesis and nanostructure fabrication, in Section 2 Section 3 presents a series of unique LAL-based techniques for nanomaterials preparation and processing These methods include LAL assisted by a temperature, electric, or magnetic field, and electrochemistry, along with PLD in liquid and laser writing of nanopatterns in liquid Many new advances in nanomaterials synthesis based on LAL are illustrated, including nanocrystal–polymer composites, doped semiconductor nanocrystals, submicrometer spherical particles, and monodisperse colloidal quantum dots (QDs) The applications of a variety of LAL-generated nanomaterials obtained from the techniques mentioned above are highlighted in Section 5, which include the fields of optics, magnetics, environment, energy, and biomedicine In Section 6, we provide an outlook on the challenges and opportunities in this promising field by elucidating the main advantages, existing problems, and future directions of LAL

2 Laser ablation of a solid target in liquid

Laser ablation of solid materials has been studied intensively for decades because it has shown great potential in laser-based materials processing including thin solid film preparation; nanocrystal synthesis; laser cutting, welding, drilling, and surface cleaning; and device fabrication [92–97] Because laser ablation of a solid material is easily carried out in a conventional deposition chamber under vacuum or filled with gas, most researchers have focused their attention on laser ablation of a

solid target in a vacuum and dilute gas because laser ablation usually takes place at a gas–solid interface [83, 84] Compared with the applications of laser ablation in vacuum or dilute gas, use of laser ablation of a solid target in a confined liquid, i.e., LAL, is comparatively limited in research considering the interactions between lasers and materials However, LAL has been widely used to prepare nanomaterials and nanostructures There are many groups that focus on this research direction globally, and a large variety of nanomaterials such as metals, metallic alloys, semiconductors, and polymers have been synthesized using LAL Therefore, in the following subsections, we consider the basic physical and chemical processes involved in

nanocrystal synthesis and nanostructure fabrication by LAL

2.1 Fundamental physical processes

We can divide the LAL-based techniques into three kinds of laser–material interaction processes on the basis of LAL applications in nanocrystal synthesis and nanostructure fabrication; i.e., microsecond-laser ablation in liquid (ms-LAL), nanosecond-laser ablation in liquid (ns-LAL), and femtosecond-laser ablation in liquid (fs-LAL) Interestingly, there are distinctly different physical processes involved in the interactions of these lasers with materials Therefore, we will discuss the fundamental principles of these processes

2.1.1 Millisecond-laser ablation in liquid and melting process

Compared with ns and fs lasers, ms lasers are rarely used to synthesize nanoparticles; instead, they tend to be used to weld and cut metals [100] Therefore, the mechanism and applications of ms-LAL remained unclear for a long time

Recently, Du et al [101] pioneered controllable synthesis of diverse nanostructures

using a ms laser with low power density Interestingly, the ms laser acts quite differently from a conventional ns laser For example, a ns laser with a short pulse width always has a power density in the range of 108–1010

W cm2 This high power density can produce a plasma plume ionized from the target In contrast, ms lasers have a low power density (106–107

W cm2) and only produce nanodroplets during LAL [43,102,103]

The mechanism of ms-LAL is mainly a heating effect Molten metal droplets are initially generated when a ms laser ablates a metal target in a liquid The temperature

of the metal droplets is so high that they can heat the liquid around them to form a vapor state in an explosive manner At this time, because of the confinement of the liquid, a high pressure is generated The boiling point of materials can be increased by the vapor-generated pressure, which means that the ablated material can maintain a liquid state even when the temperature is over its standard boiling point Because of the strong shattering effect caused by the vapor with high energy, abundant initial millimeter-sized metal droplets resulting from the ms-laser ablation of a target will be explosively ejected into nanodroplets (Fig 5a) The formation of metal nanodroplets rather than plasma has been confirmed by high-speed photography [101] Through the time-resolved images from 0.2 to 2.2 ms, many small microsized droplets are found and no plasma plume is observed (Fig 5c) Moreover, such hot metal nanodroplets are expected to react with the ambient liquid starting from its surface because of their compactness (Fig 5b) Therefore, by adjusting the liquid and laser parameters,

nanostructures with various morphologies and sizes can be produced via surface reaction [43,101]

2.1.2 Nanosecond-laser ablation in liquid and plasma formation

Yang wrote a comprehensive review about ns-LAL in 2007 The main processes

of ns-LAL include generation, transformation, and condensation of plasma plumes [15] The basic processes of ns-LAL are as follows First, plasma is generated by laser ablation of a solid target at the interface when the laser ablates the target immersed in

a liquid The plasma plume mainly results from the multiphoton absorption, ionization, and inverse Bremsstrahlung in the gaseous phase induced by laser ablation Accordingly, the plasma plume contains numerous neutral atoms, ions, and electrons from the solid This initial plasma is considered to be laser-induced plasma, because it

is directly formed by ablation of the solid target by the laser pulse (Fig 6a) Then, unlike in vacuum and gas environments, the expansion of the laser-induced plasma is strongly confined by the liquid After the solid target absorbs the later part of the laser pulse to produce a continual supply of vaporizing species, the plasma adiabatically expands to create a shock wave, which results in extra pressure and temperature in the laser-induced plasma The pressure induced by the shock wave is called the plasma-induced pressure Moreover, the temperature of the laser-induced plasma increases rapidly because of the emergence of plasma-induced pressure Accordingly, the shock wave formed by the expansion of the laser-induced plasma pushes the laser-induced plasma into a unique thermodynamic state with higher temperature, pressure, and density than those of the originally formed plasma by creating extra

temperature and pressure in the laser-induced plasma (Fig 6b) Meanwhile, four kinds

of chemical reactions occur in the laser-induced plasma and at the interface between the liquid and laser-induced plasma during the transformation of the laser-induced plasma These four chemical reactions are discussed in detail in Section 2.2.1 During the expansion and condensation of the plasma plume, it transfers energy to the surrounding liquid, which causes a thin layer of vapor to rise This vapor layer is considered to be the early stage of cavitation bubbles The last stage of evolution of a plasma plume in a liquid is cooling and shrinking accompanied with decreases in pressure and temperature because of the confinement of the liquid (Fig 6c) Finally, the plasma quenches and releases small nanoparticles (Fig 6d) A time sequence of this evolution process is also shown in Fig 6 [18]; it was visualized by fast camera imaging coupled with the shadowgraph technique [104] This image was obtained during laser ablation of a Pt wire in water, and clearly reveals the expansion and collapse process of the plasma plume (Fig 7) Note that the plasma cooling is rather fast and is accompanied with the shrinkage and vaporization of the surrounding liquid medium at the front head of the plasma plume

When the vapor layer is generated, it starts to expand and compresses the plasma back against the target more efficiently than the surrounding liquid (Fig 6c) In general, expansion and shrinkage are dynamics of cavitation bubbles The lifetime of bubble is not the fixed value, which depends strongly on laser pulse characteristics, such as laser fluence [105] The pressure and temperature in the bubble can be estimated using the hard-core van der Waals model with the bubble radius acquired

Pa, respectively [28] Note that, this state with high temperature and pressure is only observed for the initial phase; the cavitation bubble cools dramatically as it expands

The generation, expansion, and condensation of bubbles can be observed in situ

by the shadowgraph technique Fig 8 shows a bubble produced by laser irradiation of

a Cu target in water at various delay times The bubble growth and collapse is clearly seen in the time range of 400 ns to 400 μs when the laser pulse width is 19, 90, or 150

ns [106] After irradiation for 400 ns, bright emission was observed from the plasma plume generated by laser pulses of 90 and 150 ns It is obvious that a longer ns pulse results in more intense emission The bubble size is closely related to the pulse width, which indicates that the longer the laser pulse is, the larger the bubble will be in the bubble expansion process The bubble grew during the time from 1 to 100 μs, reaching its maximum radius at 100 μs, and then it shrunk until, finally, it collapsed

At this point, this system reached a thermodynamically stable state The final collapse

of the bubble released its interior mass into the liquid, including clusters and micro/nanoparticles (Fig 6e) The as-synthesized clusters and nanoparticles are unstable initially, and probably tend to agglomerate A ripening process occurs in most initial colloidal solutions synthesized by LAL

As mentioned above, the bubble originates from the thin layer of vapor around the plasma plume The vapor reaches a high temperature because the plasma transfers energy to it Similar to plasma, the vapor layer starts to expand to transform into a bubble with larger radius than that of the vapor layer The confinement of the liquid increases the pressure inside the bubble At this time, after the plasma expands and transfers its energy to the vapor layer, it gradually cools down Therefore, the bubble expands in all directions, not only moving against the liquid, but also moving against and compressing the plasma The difference in expansion behavior between plasma and bubbles is that bubbles display an expansion stage and a subsequent stage, which can be repeated several times like a damping oscillator [107] During this stage, the pressure inside the bubble gradually achieves a balance with that of the ambient liquid

At this time, the bubble reaches its maximum radius and maintains this quasi-equilibrium state for a certain period

It is considered that when the bubble collapses, nanoparticles formed as the plasma cools can diffuse in the surrounding liquid to form a colloidal solution [15] In other words, nanoparticles are mainly produced during plasma quenching Evidence for this situation is that the signals of small molecules are observed in the emission

spectra originating from the chemical reaction taking place between ablated species and species from the liquid molecules in the plasma plume [108] Thus, after the plasma condenses and collapses, the nanoparticles originating from the reaction between target species and liquid molecules will be released Then, they diffuse in the cavitation bubble, and finally release in solution

Another mechanism has been proposed that indicates laser-induced cavitation bubbles play an important role in nanoparticle formation [45,109,110] In thermodynamics, the pressure and temperature in a bubble can be calculated by

defining the bubble radius from its fast shadowgraph images Giacomo et al [28]

estimated that the temperature and pressure in a bubble can be up to 1000 K and 108

Pa based on the bubble radius However, because of the limit of instrument precision, these calculations may miss the bubble inversion phase from collapse to rebound, which could possess much higher temperature and pressure Akhatov’s theoretical model predicted that the bubble interior can reach up to 10000 K and several thousand megapascals [111] In an environment with such high temperature and pressure, phase transformation and condensation of nanoparticles seem reasonable In this sense, the chemical reaction in a bubble may be similar to that in a plasma plume In addition, the bubble duration is two orders of magnitude longer than that of plasma [18], which indicates that it exists for long enough to allow chemical reactions to proceed and nanocrystals to grow

High-time-resolution small-angle X-ray scattering has proved that nanoparticles can form in a cavitation bubble [29] This method can penetrate a bubble and identify

the species, including small particles, inside it by scattering Observations revealed that Au nanoparticles with a diameter of 8–10 nm were distributed all over the bubble, while larger ones (diameter of 45 nm) only existed in the upper part of the bubble Laser light scattering has also been used to investigate nanoparticle growth in liquids Because a probe laser beam is scattered by nanoparticles, an image of the scattered light provides the location of nanoparticles The results showed that fast growth of nanoparticles occurs until 3 μm after the laser pulse [112] A fraction of nanoparticles

is transported from the bubble into water in the expansion stage, while most remain trapped in the bubble until it collapses

The above discussion reveals that the initial stage of nanocrystal formation via chemical reaction in LAL is still controversial However, experimental evidence suggests that both plasma and bubbles are probably involved in the chemical process because of the high temperature and pressure generated The initial nucleation process may occur in the plasma plume, and a relatively long growth period could take place during bubble evolution

2.1.3 Picosecond-laser ablation in liquid and cavitation bubbles

The mechanism of a ps laser is similar to that of a ns laser [113] The reaction process can be described as follows: after the laser interacts with the solid target, hot, dense plasma is ejected from the target and its fast expansion in the surrounding liquid generates a shock wave Typically, the onset of a shock wave occurs in tens of nanoseconds, depending on the energy irradiated onto the target Behind the shock front, which rapidly moves away from the target on a time scale of hundreds of

nanoseconds, the plasma heat exchange with the liquid causes the formation of a cavitation bubble that contains vapor, gas, and nanoparticles The cavitation bubble first expands and then collapses on the time scale of hundreds of microseconds [113] Fig 9 shows shadowgraph images of shock wave and cavitation bubble time evolution in the ablation of an Au target in water by a laser with a pulse width of 25

ps and fluence of 6.3 J/cm2 It has been macroscopically shown that the interactions of

ns and ps lasers with materials are similar for both laser pulse regimes because of the identical temperature distribution within the target materials, which is caused by the short thermal diffusion length and long absorption length of ns and ps lasers [114] Although the mechanism of ps lasers is similar to that of ns lasers, the high repetition rate of ps lasers is advantageous to enhance the production of nanoparticles Barcikowski and co-workers found that the nanoparticle production at the same pulse fluence was three times higher for ps laser ablation with higher repetition rate compared with fs lasers at the same laser fluence [115] Using a ps pulse duration, higher production by one order of magnitude was realized compared with that of a conventional fs-LAL Moreover, the production of ligand-free nanoparticles and gold−ssDNA nanoconjugates with the same nanoparticle characteristics, surface composition, biomolecule loads, and the same level of biomolecule structure integrity

by fs-LAL as those achieved by ns-LAL has been attained [46] Recently, many reports have proven that compared with ns and fs lasers, ps lasers are superior to improve the production of nanoparticles synthesized by LAL, even up to a pilot-scale process [32,33]

2.1.4 Femtosecond-laser ablation in liquid and multiple processes

The interactions between fs laser pulses and materials are controlled by different mechanisms to those of ns and ps lasers with materials [116,117] In this study, we divide these processes into four categories based on the incident power density: phase explosion, fragmentation, Coulomb explosion, and plasma ablation

During fs laser irradiation, after photon energy is absorbed by carriers in the materials, free electrons of metals, electrons in the valence band of semiconductors or insulators, and multiphoton ionization provide the initial electron kinetic energy to produce additional free carriers through an impact ionization avalanche process Fig

10 shows the simulation for fs ablation in an ambient environment When the incident power density is below a threshold of ~1013 Wcm−2, under fs irradiation (~5×1011Wcm−2), about 0.7 ps after the beginning of the pulse, a hot (~8000 K) and highly pressurized (~10 GPa) liquid layer forms on the liquid surface (Fig 10a) The more solid target material is transformed into a liquid metal because of the propagation of the melt front further into the bulk (Fig 10b) The relaxation of the pressure within the liquid layer causes the upper layer to rapidly expand, resulting in the formation of

a void (Fig 10c) The void rapidly grows (Fig 10d), and finally ejects a large amount

of materials (Fig 10e) This process is called phase explosion, and occurs on a time scale of ~10−12–10−10 s The solidification of the non-ablated molten material subsequently takes place in 10−11–10−9 s (Fig 10f) If the power density is increased

to 1.1×1012 Wcm−2, the ejected matter has a diffuse, clusterlike structure, which is known as the fragmentation process (Fig 10g) [118,119]

The above case is simulation results on fs laser ablation in vacuum Recently,

some simulation results based on fs-LAL have been developed [120,121] Shih et al

report the results of first atomistic simulations of laser ablation of metal targets in liquid environment They show a fully atomistic description of laser interactions with metal targets, and acoustic impedance matching boundary conditions for simulation of laser ablation of a thin silver film deposited on a silica substrate [120] The simulations, performed at two laser fluences in the regime of phase explosion, predict

a rapid deceleration of the ejected ablation plume and the formation of a dense superheated molten layer at the water-plume interface The water in contact with the hot metal layer is brought to the supercritical state and transforms into an expanding low density metal-water mixing region that serves as a precursor for the formation of

a cavitation bubble The dynamics of the ablation plume expansion is strongly influenced by the presence of the liquid environment The snapshot of atomic configuration produced in the simulation of LAL is presented along with normalized density distributions plotted for both silver and water, as shown in Fig 11 [120]

In addition, Roeterkink et al [122] performed a fs laser experiment in

combination with time-of-flight (TOF) spectroscopy of a Si wafer They found that in the TOF spectra, the fast Si+ peak corresponded to a velocity half of that observed for

Si2+, which indicates that a Coulomb explosion occurred At this time, if the power density continues to increase, plasma ablation will be observed (Fig 12a)

Therefore, when the power density is increased to very close to the ablation threshold value, a Coulomb explosion happens First, materials absorb the high energy

supplied by the laser pulse (Fig 12b) Second, electrons are stripped from the atoms via photoelectric and thermionic emission (Fig 12c) Thus, an electric field with very high intensity is produced on the surface of the irradiated area This electric field will result in a very strong repulsive force between positive ions Because the repulsive force is larger than the bond strength, the surface of the solid material is stripped [123], as shown in Fig 12d

Hashimoto et al [124] performed a numerical simulation to explain the

mechanism of the laser-induced size decrease of aqueous gold nanoparticles using ns and fs lasers based on the two-temperature model considering electron temperature, lattice temperature, and the temperature of the medium surrounding the particles Their results showed that the fs laser offered adequate energy to raise the electron temperature up to 7000 K for liquid gold and above 8000 K for solid gold, which meets the requirement for Coulomb explosion In contrast, the photothermal mechanism is responsible for the case of the ns laser As well the theoretical estimation, they also performed in situ extinction spectroscopy and transient absorption spectroscopy of the fs laser-induced fragmentation of aqueous gold nanoparticles [125] This study was the first direct spectroscopic observation indicating that Coulomb explosion causes nanocrystal fragmentation If the power density increases above the plasma ablation threshold, a direct solid-to-plasma transition is followed by optical breakdown [126,127], in which materials are fully ionized and evaporated to form a plasma with high temperature and density

Although ns and fs lasers both produce plasmas, the parameters of these two

types of plasmas are not the same Fig 13a and b show the temporal behavior of the plasma temperature and electron density of ns and fs laser-induced plasmas, respectively [128] Compared with the ns plasma, the density and temperature of the fs-derived plasma are lower and decreased faster because the fs laser did not display laser–plasma interactions Fig 13c and d depict the time-dependent perpendicular and lateral expansion distances for the ns and fs laser-induced plasmas The perpendicular expansion distance of the shock wave produced by the ns laser is proportional to t2/5, which is like spherical propagation, while that produced by the fs laser is proportional

to t2/3, conforming to one-dimensional expansion Initially (<1 ns), the fs laser-induced plasma expands mainly in the direction perpendicular to the target surface After several ns, the fs laser-induced plasma expands in both the lateral and perpendicular directions The expansion in the perpendicular direction is faster than that in the lateral direction In comparison, the ns laser-induced plasma expands in both directions at similar speed Therefore, there are marked differences between the

plasmas generated by ns and fs lasers

2.2 Fundamental chemical processes

2.2.1 Chemical reactions in plasma

During the transformation of plasma, four representative kinds of chemical reactions occur inside the laser-induced plasma and at the interface between the plasma and liquid Importantly, the confinement of the liquid drives the laser-induced plasma into a thermodynamic state that definitely differs from that of laser ablation in

a gas environment The liquid always takes part in the chemical reactions Therefore,

these chemical reactions are different from those that occur in vacuum and gas environments [15] Sakka et al [108] researched the solid–liquid interface in LAL using emission spectroscopy They used graphite and boron nitride targets, and water, benzene, n-hexane, and carbon tetrachloride as liquids to perform LAL experiments

In the spectra of the boron nitride–water system, the presence of BO molecules became apparent 40 ns after irradiation began Because there was no oxygen source in the boron nitride crystal, the oxygen atom in this molecule must have originated from water Also, in the spectra for boron nitride in benzene, n-hexane, and carbon tetrachloride, the presence of C2 and CN was detected Similarly, the carbon atoms in C2 and CN should originate from the liquid The presence of BO and CN molecules is evidence for the reaction between the species ablated from the solid surface and the species present in the liquid

Four kinds of chemical reactions occur in the laser-induced plasma and at the interface between the liquid and laser-induced plasma during the transformation of the laser-induced plasma Table 1 summarizes these reactions including their location, species, and products

(i) The first kind of chemical reaction takes place inside the laser-induced plasma Because the laser-induced high-density plasma is in a state with high temperature and high pressure, metastable phases are always expected to appear because of the high-temperature chemical reactions between ablated species from the target (Fig 14a) Representative chemical reactions include a hexagonal carbon phase that changes into a cubic carbon phase, and the conversion of a zinc-blende Si phase

into face-centered cubic (fcc) Si [129–132]

(ii) The second kind of chemical reaction also takes place inside the laser-induced plasma The reactant species are not only from the target, but from both the target and liquid The high temperature and high pressure in front of the laser-induced plasma result in the excitation and evaporation of the liquid molecules at the interface between the laser-induced plasma and liquid, and create new plasma from the liquid molecules at the plasma–liquid interface Because this plasma is generated by the laser-induced plasma, it is called plasma-induced plasma Once it

is produced, it will quickly mix with the original laser-induced plasma Therefore, chemical reactions occur between the species from the laser ablating target and the species from the excited liquid molecules inside the laser-induced plasma (Fig 14b) The representative chemical reactions are formation of various oxides, nitrides, and carbides [65,82,133]

(iii) The third kind of chemical reactions take place at the interface between the laser-induced plasma and liquid The laser-induced plasma with high temperature, pressure, and density is suitable to induce high-temperature chemical reactions between the ablated species from the target and liquid molecules (Fig 14c) A representative chemical reaction is the generation of hydrated oxides during LAL [91,134]

(iv) The fourth kind of chemical reactions occur inside the liquid The ablated species from the solid target will be ejected out from the laser-induced plasma into the liquid by the extremely high pressure At this point, the chemical reactions

between the ablated species and liquid molecules will take place inside the liquid (Fig 14d) A representative chemical reaction is the formation of hydroxides [135]

Therefore, it is shown that three of the four kinds of chemical reactions involve two species that originate from the solid target and liquid If appropriate targets and liquids are chosen, various nanomaterials can be controllably synthesized inside the

plasma, at the plasma–liquid interface and inside the liquid Shafeev and colleagues

chose different kinds of liquids, such as ethanol, ethanol with saturated H2, and dilute alkaline solution, to study the chemical reactions between these liquids and aluminum [136] Their results showed that aluminum nanoparticles were generated by LAL of bulk Al in ethanol using a fs or ps laser source These nanoparticles were mostly amorphous with single-crystalline inclusions and a native oxide cladding However, if the liquid was ethanol with saturated H2, the products changed to aluminum nanoparticles with cavities inside them originating from the release of hydrogen during the solidification of aluminum nanoparticles [137] This study described the effect of the presence of a light gas dissolved in the liquid media during LAL for the first time [137,138] Interestingly, the same group reported a new type of self-organization of gas bubbles formed during etching of an aluminum target in a dilute alkaline solution [139,140] The structures formed because of the rise of hydrogen bubbles emitted from the liquid surface in the course of a chemical reaction between the metal and ammonia solution An aqueous ammonia solution is a weak base, and its chemical interaction with aluminum leads to emission of hydrogen After

a period of time, the bubbles formed a stationary pattern that depended on the geometry of the etched area Special laser processing of the etched area shaped as a vortex led to rotation of the liquid without additional applied mechanical action This bubble self-organization resulted from the unique combination of laser irradiation of metals and relatively slow chemical etching of aluminum [140]

2.2.2 Chemical reactions at the interfaces between solid and liquid

Different from the chemical reactions in plasma and bubbles, surface reactions including the Kirkendall effect are proposed to occur during ms-LAL The Kirkendall effect has been used to form hollow nanoparticles of metal oxides and sulfides [141,142] This effect describes a chemical process during oxidation of a metallic particle in which its metal core diffuses rapidly outward while the oxidizer diffuses slowly inward, which can lead to the synthesis of hollow nanoparticles [143]

As mentioned above, a laser with a long pulse width like a ms laser with low power density (106 Wcm−2) often generates nanodroplets rather than plasma [43,102,103] As a result, surface reactions are expected, which can give rise to core–shell nanoparticles Here we consider the formation of ZnS nanoparticles by ms-LAL as an example [43] Zn nanodroplets are ejected after ms-laser ablation of the

Zn target These “hot” nanodroplets contact with the sulfur-rich liquid, such as dodecyl mercaptan/n-hexane Because of the high temperature at the interface between the Zn nanodroplets and liquid, the liquid molecules tend to react with Zn Therefore, the surface of Zn nanodroplets is vulcanized and gradually transformed into ZnS; in other words, Zn/ZnS core–shell nanoparticles form Because of the high

diffusion coefficient of the Zn core and low diffusion coefficient of the S shell at high temperature, the core–shell Zn/ZnS nanoparticles gradually turn into ZnS hollow nanoparticles A ms laser can provide a high power density to rapidly heat and ablate

a target, after which the cold liquid medium quenches the high-temperature phase to form nanoparticles The transformation is so quick that oxidation and sulfuration cannot proceed inside the core and thus core–shell nanoparticles are produced Fig 15a–e show the ZnS hollow particles observed at different stages and corresponding structural models Based on the same route, Mg nanoparticles have also been transformed into MgO hollow nanospheres by ms-laser irradiation, indicating the faster outward diffusion of Mg atoms and slower inward diffusion of O atoms [144] Therefore, the Kirkendall effect induced by ms-LAL can be applied to oxides and sulfides

Hollow nanostructures can also be synthesized using other types of lasers However, their formation mechanism is different from that using a ms laser Chrisey

et al [74] reported the fabrication and formation mechanism of hollow MgO particles

by a ns excimer laser The formation mechanism of MgO nanoparticles is different from that of the hollow particles synthesized by a ms laser As we discussed above, the ms laser is mainly involved in surface reactions However, the MgO nanoparticles synthesized by a ns laser are considered to be generated on laser-induced bubbles by bubble surface pinning There are two routes for this process First, the clusters are formed in the bubble and accumulate at the bubble–liquid interface as the bubble oscillates, especially during the collapse time Second, the clusters are dispersed in the

liquid around the bubbles, including the laser-induced bubbles and metastable ultramicroscopic bubbles originating from collapsed bubbles Once a bubble–liquid interface absorbs enough clusters, they begin to encounter each other, and a cluster network or layer will form Finally, the clusters may produce a hollow aggregate or even melt to form a smooth layer, which could provide a template for further nucleation and growth of a hollow particle with a thicker shell

The same mechanism also applies to the formation of Al2O3 hollow nanoparticles synthesized by a similar ns laser [145] The excimer laser ablation produced nanoclusters from the target and bubbles from the liquid The bubble–liquid interfaces trapped the nanoclusters, resulting in the formation of hollow particles

Shafeev et al [138] produced aluminum hollow nanoparticles by laser ablation of an

aluminum target in liquid ethanol saturated with hydrogen They found that the aluminum nanoparticles had cavities inside them because of the release of hydrogen during their solidification Therefore, only hollow structures synthesized by a ms laser are influenced by heat and the Kirkendall effect

In this section, we explored the physical processes and chemical reactions that occur during LAL using ms, ns, and fs lasers The results show that ablation mechanisms differ markedly when lasers with different pulse widths are used in LAL; i.e., molten droplets for ms lasers, plasma and bubbles for ns lasers, and multiple processes including phase explosion, fragmentation, Coulomb explosion, and plasma ablation for fs lasers Because of their different ablation processes, each type of laser has its own set of advantages and range of applications Therefore, obtaining a

comprehensive understanding of the fundamental mechanism of LAL is of benefit to design and control nanocrystal synthesis and nanostructure fabrication

3 LAL-based nanomaterials preparation and processing

According to the above descriptions of the basic physical process and chemical reactions during LAL treatment, LAL can be expected to be advantageous for the synthesis of nanocrystals For example, small nanocrystals should be obtained because of the rapid quenching time In addition, the nanocrystals should possess a clean surface because of the lack of complex precursors Metastable phases that preferr a non-equilibrium thermodynamic state with high pressure and high temperature can also be accessed Therefore, in this section we discuss how to promote LAL-based nanomaterials preparation and processing by the assistance of an external field, such as an electric, temperature, or magnetic field, to synthesize novel nanocrystals PLD in liquid and laser writing of functional nanopatterns in liquid for the production of nanoparticle patterning and manufacture of functional nanostructures are also described

3.1 Electric field-assisted LAL for shape control of metal oxide nanocrystals 3.1.1 Metal oxide semiconductor nanocrystals with various shapes

Germanium dioxide (GeO2) nanostructures are widely used in optoelectronics because GeO2 is a high-k dielectric with favorable phase stability [146,147] GeO2 nanostructures with controlled shape and morphology are commonly synthesized by the sol–gel reaction However, in this method, substantial quantities of capping agents

are attached to the surface of the GeO2 nanostructures [148,149], which may degrade their performance To address this issue, the unique approach of EFLAL, which does not need any catalyst or organic additives, has been developed for the controllable fabrication of GeO2 micro- and nanocubes, and nanospindles with high-index facets [62] A schematic illustration of the experimental setup for EFLAL and external electric field acting on the plasma are shown in Fig 16a and b, respectively The difference between common LAL and EFLAL is that in EFLAL, a direct-current (DC) electric field with adjustable voltage from two parallel electrodes is applied on both sides of the target The samples synthesized by EFLAL exhibited two different morphologies: nanocubes consisting of {1011} high-index facets (Fig 16c) and nanospindles (Fig 16d) The different morphologies are related to the applied electric field Nanocubes are obtained under a voltage of 14.5 V, while nanospindles form at

32 V Interestingly, a red-shift of emission wavelength is observed as the sample shape evolved from cube to spindle; i.e., shape-dependent luminescence was exhibited (Fig 16e)

3.1.2 Influence of electric field on the shape of nanocrystals

For comparison, the same experiment without an external electric field was carried out to prove that the applied electric field allowed shape control The control experiment without an external electric field gave spherical particles, consistent with related reports [150,151] Therefore, the application of an electric field during laser ablation of Ge has a marked effect on the formation of high-index facets nanocubes and nanospindles In addition, the spherical particles were Ge rather than GeO2

Some researchers have obtained similar nanostructures without the need for an external electric field [152,153] This raises the question of whether the electric field plays a crucial role in the formation of these nanostructures Therefore, we compared

similar structures and their synthesis conditions in to try to make basic assumptions and propose a mechanism for nanostructure formation Khan et al [153] generated NiO nanoparticles using a high-power, high-brightness continuous-wave fiber source with a wavelength of 1070 nm The shape of the nanoparticles was affected by the surfactant SDS; nanoparticle shape changed from spherical in water to tetragonal with increasing SDS concentration Most of the particles were tetragonal when the SDS concentration was 0.1 M According to selective adsorption theory, a surfactant or mixture of surfactants are selectively adsorbed at different crystallographic faces of growing crystals This modifies the surface free energy of individual crystallographic faces and leads to shape anisotropy in the resulting crystals Wang and colleagues reported ovoid-like nanostructures of YVO4:Eu3+ polycrystals that were composed of many smaller nanoparticles, which were attributed to oriented attachment [152] Conversely, in our GeO2 study described above, we found that an electric field was required to obtain nanocubes and nanospindles; spherical particles formed without an external electric field This means the electric field plays an important role in governing nanoparticle shape In contrast, the first example above used a surfactant to change particle morphology, while the second study produced polycrystalline rather than single crystalline particles It is worth noting that we did not add any surfactant

to our precursor; only the Ge target and deionized water were used Therefore, our results cannot be explained by the selective adsorption theory Besides, the GeO2 nanoparticles in our samples are single crystalline rather than polycrystalline Therefore, we conclude that the external electric field does have an important

influence on particle shape

3.2 Electric field-assisted LAL for nanostructure fabrication

3.2.1 Fabrication of functional metal oxide nanostructures

Our group has not only used an electric field to control the shape of nanocrystals formed by LAL, we have also used an electric field to assemble novel nanostructures from isolated nanocrystals in one step [63] Section 3.1 focused on the growth of single-crystalline nanoparticles, such as the influence of an electric field on the crystallographic growth direction In Section 3.2, we consider how to assemble polycrystalline nanoparticles into ordered nanostructures Taking the synthesis of CuO nanospindles as an example, we introduce the relevant studies In EFLAL, when

a laser ablates a Cu target in deionized water, at the same time, a DC electric field with adjustable voltage is applied on both sides of the reaction cell The resulting samples are polycrystalline nanospindles composed of many smaller nanostructures consisting of rod-like and spherical particles (Fig 17a and b) Importantly, the optical absorption peaks of these nanospindles depend on their morphology, as shown in Fig 17c Clearly, only one broad peak located at about 400 nm is observed in the sample fabricated without an applied electric field, similar to that in a previous report [154–156] Two broad peaks appear in the absorption spectra of the samples synthesized at 40, 80, and 120 V These absorption peaks can be separated into two groups consisting of those at relatively long and relatively short wavelengths Two peaks are observed because the optical absorption depends on both the polarization and energy-band structure of the nanocrystals, which are closely related to their shape

and size Thus, the appearance of two peaks indicates that the size and morphology of the CuO nanospindles is altered under different applied fields

3.2.2 Oriented attachment mechanism of nanostructure fabrication

Oriented attachment and aggregation is proposed to be responsible for the formation of CuO nanospindles during EFLAL above [119, 120] In this mechanism, large particles grow from primary small ones by oriented attachment, where adjacent nanoparticles are self-assembled by sharing a common crystallographic orientation and particles combine at a planar interface to lower the overall energy of the system [63,157–161] This proposed mechanism is supported by the transmission electron microscopy (TEM) observation that the as-prepared CuO nanospindles contain many units that align in a certain direction The following fabrication process is proposed First, the CuO nanocrystals are synthesized by LAL (Fig 17d), then rod-like CuO nanostructures form by aggregation of the primary nanocrystals through an oriented attachment mechanism (Fig 17e) Finally, nanospindles form from the CuO nanocrystals and rods as building blocks, assisted by the applied electric field (Fig 17f) Thus, it is suggested that the electric field plays a vital role in the self-assembly

of nanospindles Wang et al [152] fabricated YVO4:Eu3+ polycrystalline nanostructures with a similar morphology by LAL without an applied electric field This means that for some specific metal oxide nanostructures, the spindle shape forms easily through the oriented attachment mechanism However, in our CuO nanostructures, when the strength of the applied electric field is increased, the shape

of the nanostructures changes, especially the average ratio of the length to the

diameter of the nanospindles To confirm the morphology change of CuO with external electric field, we used ultraviolet–visible (UV-vis) spectroscopy to measure the optical property change with applied electric field because the shape and size of nanostructures strongly influences their optical properties In other words, oriented attachment occurs in the reaction process induced by LAL, and electric field enhances this process The reason why such oriented assembly can be achieved is that CuO is a polar metal oxide that is easily polarized under an applied electric field As a result, CuO is aligned in the applied field direction because of the dipolar interaction induced

by the electric field, which is one of the driving forces for nanocrystal aggregation [159] This EFLAL approach is anticipated to be useful to fabricate functional structures composed of nanoscale building blocks

3.3 Room temperature ripening-assisted LAL for nanostructure fabrication 3.3.1 One-and two-dimensional nanostructure fabrication

The products synthesized by LAL without an external field always possess sphere-like morphology, including noble metals, oxides, sulfides, and selenides

[50,162–164] This is because a spherical morphology has the minimal surface area at

the same volume in the quenching process Recently, it was suggested that by aging at room temperature, the initially spherical nanocrystals formed LAL will spontaneously assemble into various nanostructures, such as one-dimensional nanowires and two-dimensional nanoflakes [60,61]

Xiao and co-workers established temperature field-assisted LAL for nanostructure fabrication [60,61] In the case of MnOOH nanowire fabrication, a