Femtosecond pulsed laser action on two-dimensional nanosheet actuators at van der Waals interface

Scientists at the Chinese Academy of Sciences used femtosecond pulsed lasers on two-dimensional nanosheet actuators at the van der Waals interface, The associated studies, Photoacoustic 2D actuator via femtosecond pulsed laser action on van der Waals interfaces, were published in Nature Communications.

The realization of optically controlled nanomachine engineering can meet the requirements of non-contact and non-invasive in optoelectronics, nanotechnology and biology. Conventional optical operations are based primarily on optical and phoresis forces, which typically drive particles in a gas or liquid environment. However, developing optical drives in non-fluid environments such as strong van der Waals interfaces remains difficult. Here, the researchers describe an efficient two-dimensional nanosheet driver directed by an orthogonal femtosecond laser, in which two-dimensional VSe2 and TiSe2 nanosheets deposited on a sapphire substrate can overcome interfacial van der Waals forces (tens and hundreds of megapascals of surface density) and move on a horizontal surface. The researchers attribute the observed optical drive to laser-induced asymmetric thermal stress and momentum generated by sound waves on the inner surface of the nanosheet. Two-dimensional semi-metallic materials with high absorption coefficient enrich the material family suitable for realizing photocontrolled nanomachines on the plane.

The researchers report interesting properties of two-dimensional materials under ultra-fast laser exposure, in which VSe2 and TiSe2 nanosheets are driven by femtosecond pulsed lasers on dry contact with flat sapphire and quartz glass substrates, showing that large vdW interactions between nanosheets and substrates are overcome. In addition, the researchers studied the properties of various two-dimensional materials to find the sensitive parameters in the optical drive. The researchers have analyzed the possible mechanism in terms of the magnitude of the force acting in it, and have proposed an understanding of the nature of the discovered phenomenon in terms of the photoacoustic mechanism. As observed by transmission electron microscopy, the air gap between the edge of the nanosheet and the substrate inevitably leads to local overheating, which leads to asymmetric thermal stress and surface acoustic waves. The large absorption coefficient (~ 105 cm−1) appears to be the key characteristic of this optical drive through analysis of different two-dimensional material - substrate systems. For such materials, the coefficient of thermal expansion and bulk modulus also become important. The maximum velocity efficiency of the VSe2 actuator observed in the experiment is 434 μm·s−1·mW−1, which is much higher than the photoacoustic operation efficiency reported previously. Therefore, the researchers propose a method to overcome the adhesion between two-dimensional materials and substrates, expanding the application possibilities of two-dimensional materials as nanomachines.

The VSe2 nanosheets were attached to a horizontally polished sapphire substrate by mechanical stripping (ME) method and completely covered by a vertically pulsed laser beam, as shown in Figure 1a. When the pulsed laser irradiated, the VSe2 nanosheets began to move and continued to move in the uniform illumination region. The light source is then moved in the direction of the nanosheet to make it move evenly. The motion characteristics are shown in Figure 1b. Because of the connection with the substrate, the motion of the nanosheet is limited to the surface planes of the substrate (x, y planes). This light-induced motion indicates that a conversion of light to mechanical energy occurs in the actuator system.

Figure 1: Optical drive of a two-dimensional material nanosheet on sapphire substrate.

To further assess the impact of the exercise, the researchers determined the speed of the VSe2 nanosheet. Figure 2a shows the travel speed as a function of repetition frequency when the pulse energy is fixed. The speed increases linearly with the repetition rate, indicating that each laser pulse can drive the nanosheet to move a certain distance (Figure 2b), and the movement induced by each pulse is independent of each other. Figure 2c shows the relationship between the velocity and the energy of the laser pulse. It increases linearly with laser energy (FIG. 2d) and in particular has stepped thresholds.

Figure 2: VSe2 nanosheet driver speed.

One of the nanosheets was cut together with the sapphire substrate using focused ion beam (FIB) technology, and the contact area was observed using a high-resolution transmission electron microscope, as shown in Figure 3.

Figure 3: Transmission electron microscope cross section image of VSe2 nanosheet on sapphire substrate.

Figure 4: Temporal and spatial distribution model of VSe2 nanosheet temperature increment after laser pulse.

Figure 5: Optical drive of VSe2 nanosheet at 1hz.

The full optical drive of two-dimensional nanosheets in non-fluid environment is realized, which is instructive for its practical application. For example, as shown in Figure 6, the separated nanosheets gradually move towards the larger nanosheets until they are successfully spliced together. Thus, the photoacoustic manipulation method allows researchers to assemble two-dimensional materials together and construct their structures, which is a promising way to manufacture transverse photonic and optoelectronic devices.

Figure 6: All-optical splicing of VSe2 nanosheets.

Link to paper:

https://www.nature.com/articles/s41467-023-37763-8

Source: Yangtze River Delta G60 Laser Alliance