RHEED analysis of the oxidized M′₂M^″_xX_yene sheets by ablated plasma thrust method in pulsed laser deposition chamber

The discovery of high-temperature superconductivity is the most awaited moment for material scientists. MXene has recently been the most promising material in several aspects of graphene. MXene has the general formula M_n+1X_n, with n = 1, 2, and 3, derived from the MAX phase, where M is an early transitional metal, X is carbide or nitride, and A is the A block element.¹ We have used Mo₂Ti_xC_yene (x = 1 for y = 2 and x = 2 for y = 3; Mo and Ti are the outer and inner transitional metal layers), the ordered double transitional metal layered carbides for their oxidation.^2,3 However, there are many challenges in synthesizing the oxides of MXenes due to their sensitive layered properties.⁴ For growing their oxides and analyzing their surface properties, Pulsed Laser Deposition (PLD) and Angle-Resolved Photoemission Spectroscopy (ARPES) are the most powerful and appropriate techniques. The Ablated Plasma (AP)-thrust method, as a new concept, is adopted for the oxidation of the MXene surface. In this method, the MXene material used as a substrate is oxidized with atomic oxygen that is derived from the oxygen molecule filled as background gas in the PLD chamber. The energy needed for the atomicity of oxygen is provided by the thermal energy of the ablated plume in the form of plasma^5,6 originated from the target. The proper ionization and thrusting by the plume at a stopping distance nearly equal to the substrate target distance leave an appropriate environment for the oxidation. The atomic oxygen will occupy the hollow site² above the carbon atoms in the double-ordered MXenes. The oxidized sample surface can be checked by Reflective High-Energy Electron Diffraction (RHEED) and fast photography (ICCD—Intensified Charge Coupled Device) approaches. The ambient gas pressure for oxidation is found to be 0.1 and 0.2 mbar for an input laser energy of 400 mJ, an energy density of 2 J cm⁻², 300 °C, and a pulse repetition rate 2 Hz. The oxidized MXene can be cleaved in situ through the proper crystallographic axes for its flat and plane surface for the ARPES test requirement. Theoretically, it has been shown that the oxidized MXene has topological insulating properties.⁷ We are expecting its experimental verification.

PLD is one of the effective (simple and fast) techniques used for thin film deposition and nanomaterial synthesis. It can produce a scalable sample in 10–15 min. Laser technology used in PLD has made it more effective in depositing complex stoichiometry multi-element and multi-layered oxides, nitrides, superlattices, etc.⁸ Materials such as YBa₂Cu₃O_7−δ (YBCO) single crystals, without a natural cleaving plane, have problems in their characterization. This paper focuses on MXene oxide synthesis by AP-thrust PLD. As discussed theoretically, the surface of an (Ordered Double Transition Metal Layered) ODTML MXene has hollow sites above the carbon atom with a required formation energy of 4.40 eV for Mo₂TiC₂O₂, and the formation energy for Mo₂Ti₂C₃O₂ is a little bit higher than that.⁹ For a 2 × 2 × 1 supercell, the formation energy of oxygen vacancy is obtained as follows:

where E(X) is the total energy of system X.¹⁰ The oxygen formed by the plasma plume occupies the site and forms MXene oxide.

The plasma plume expansion in vacuum was studied by Anisimov et al.¹¹ and Singh and Narayan,¹² while that in background gas was described by Predtechensky and Mayorov (external dynamics of the contact front)¹³ and Arnold et al. (internal dynamics and background gas).¹⁴ The highest deposition rate of PLD is of the order of 10²⁰ atoms cm⁻² s⁻¹.^15,16 This gives the higher degree of supersaturation according to the following equation:

where κ_B, R, and R_o are the Boltzmann constant, actual deposition rate, and rate at equilibrium temperature T. In our case, the adsorption of oxygen is considered. So, we are concerned with the adsorption rate rather than the growth rate. Instead, it is a strong reference for us.

The Mo₂TiC₂ and Mo₂Ti₂C₃ sheets were used as substrates for oxidation in the PLD system. The PLD system used at the RRCAT, Indore, was Lambda Physik Complex 201 where we used a bulk aluminum target¹⁷ of 1^″ diameter and 1–2 mm thickness. The delaminated M′₂M^″_xX_yene substrate can be adjusted from room temperature to 850 °C, with a repetition rate in the range of 1–10 Hz and a maximum pulse energy of 600 mJ, and the target to substrate distance^18,19 can be varied from 4 to 10 cm. It uses a high-power KrF excimer (∼248 nm, ∼20 ns, ∼10 Hz, and ∼108 Wcm⁻²) to ablate the rotating target material. It can be adjusted with operating wavelengths of 992, 496, and 248 nm with a pulse duration of nearly 8 ns with the help of a delay generator. The energy density of the laser is controlled by adjusting the mask size and magnification. A multitarget deposition chamber with six targets deposits thin film with a computer-controlled system. Materials such as complex ceramic oxides, high-temperature superconductors, Heusler’s alloys, and silicon carbides can be easily treated in this chamber. The plasma plume is recorded using an ICCD camera (only data are taken here without images), and the deposition on the substrate was recorded by high-pressure RHEED. We can observe all these phenomena in the PLD chamber through the view port as shown in Fig. 1. The adsorption site of oxygen on the surface is the hollow site above the carbon atom, as shown in Fig. 2, of the trigonal-hexagonal crystal structure with space group $P3̄m1$⁠. Figures 2(a) and 2(b) show the top and side views of Mo₂TiC₂ and Mo₂Ti₂C₃ and their oxides, respectively. The sources of oxygen in the chamber are (a) background oxygen, (b) target or plasma oxygen, and (c) substrate oxygen.²⁰ The study by Chen et al. on O-18 claimed that the oxygen in the substrate was predominant on increasing background oxygen pressure.²¹ The measuring device was focused 1 mm apart from the target, and an aluminum spectral line at 281.62 nm was taken for density calculation.

At the laser wavelength of 248 nm, an energy fluence of 2 J cm⁻², an exposure time of ∼8 ns, and a substrate to target distance of 4 cm at room temperature, the different sets of data were recorded with the help of the ICCD camera at different times and pressures. They were then plotted in a graph as shown in Fig. 3. Initially, at a time of ≤1 µs, a position of ≤1 cm, and a velocity of 12 km/s, the expansion is nearly linear. At around 2 cm, the expansion breaks due to the oxygen pressure. At lower pressure (p = 0.05 mbar), the plume easily reaches the substrate after 10 µs. However, at higher pressure (p = 0.2 mbar), the plume does not reach the substrate even at 20 µs and ultimately stops showing the snow-plough effect after the energy of ablated species is transferred to the background gas.

The graph of plume front velocity (u) against perpendicular distance from the target surface R is shown in Fig. 4. In this figure, the velocity decreases sharply along with the distance profile due to the effect of background pressure. The graph of the distance traveled by the plume front against time at three different substrate temperatures and the plot of substrate temperature and corresponding plume front velocity are shown in Figs. 5 and 6, respectively.

In this case, the background pressure is kept constant, say, at p = 0.2 mbar, and the plume front expansion is observed for substrate temperatures of 300, 600, and 900 °C with the help of ICCD images. The related data were recorded and are plotted in a graph as shown in Fig. 4. This figure shows that there is a significant increase in the velocity (kinetic energy) of the contact front with higher substrate temperature. The rate of change increases for higher temperatures, i.e., up to 300 °C, and the change was ≈0.03–0.05 eV, and from 800 to 900 °C, the change in the energy was ≈0.2–0.3 eV. The plot of substrate temperature against corresponding plume front velocity is shown in Fig. 5. The results are similar to those of the previous one.

An Yttrium Aluminum Garnet (YAG) excimer is used, and we recorded the data obtained from the images captured by fast photography (ICCD camera) at first 0–30 ns under the effect of different wavelengths with different energies on the interaction that takes place in the process of adsorption.^22,23 The fundamental (996 nm), second (496 nm), and fourth harmonic (248 nm) wavelengths with a full wave half maximum of 6 ns were focused onto the 2 mm thick aluminum target (spot size, 100 mm) through a plano-convex quartz lens to produce plasma. The laser pulse energies were 10, 20, and 40 mJ, respectively. Their values per unit cm² are 153, 318, and 636 J, respectively. The energy after the explosion is 10%–15% of the input laser energy.^24,25 

The distance–time plot for the spherical plume (n = 3) for wavelengths of (a) 248 nm, (b) 496 nm, and (c) 992 nm is shown in the Figs. 7(a)–7(c) respectively. The curves obtained for laser energies of 10 and 20 mJ are fitted well on spherical Taylor–Sedov solution,²⁶ which agree well with the input laser energies. The plasma generated by a longer wavelength of 996 nm with a higher energy of 40 mJ is more cylindrical than that generated by shorter ones. This is due to the phenomenon of the Inverse Bremsstrahlung (IB) process induced by longer wavelength radiation. If the process was carried by a shorter wavelength, then the process is bremsstrahlung due to photoionization (PI) as studied by Chang et al. in 1996.^23,27 The photoionization process involving higher temperature does not work in our surface-sensitive substrate. So, the input laser with a longer wavelength and larger energy fluence is appropriate for the adsorption process.

Similarly, the R–t plot for a pulse energy of 10 mJ at 992, 496, and 248 nm laser ablation (obtained from ICCD images) is shown in Fig. 8. The best fits given in Fig. 8 are obtained using the drag model. The blast model is not fitted. The result agrees well for 992 nm but deviates for 248 and 496 nm early just opposite to the previous case.

The temporal advancement of the electron density comparison for the three wavelengths of 248, 496, and 992 nm for each energy is shown in Figs. 9(a)–9(c). The electron density decreases abruptly for all wavelengths and energies. This is due to the background oxygen. As the ablated plasma plume pressure and background pressure at around 200 ns become equal, all the wavelengths of different energies decayed to increase the electron number density (4 ± 0.5 × 10¹⁶ cm⁻³). The electron density at 992 nm was the lowest and that at 248 nm was the highest. Moreover, the increase in laser energy increases the electron number density. The highest density for 10 and 40 mJ is 5.5 × 10¹⁷ cm⁻³ and 1.2 × 10¹⁸ cm⁻³ for 248 nm and 4.2 × 10¹⁷ cm⁻³ and 1.18 × 10¹⁸ cm⁻³ for 496 nm, respectively. The density of plasma generated by a longer wavelength of 992 nm is not affected significantly.

For the laser energy of 100 mJ, the pressure of 0.26 mbar, and the substrate–target distance of 4.5 cm, a set of data for the density of the atoms at different contact front distances is taken and plotted in the graph as shown in Fig. 10. In this figure, the density of oxygen is relatively low and is almost independent of the distance from the target to the contact front, while that for the ablated aluminum vapor slowly decreases with the distance. Similarly, the rate of decrease in temperature for vapor is higher than that for oxygen, which decreases rapidly than the rate of decrease in temperature of oxygen with the target substrate distance as shown in Fig. 11. The temperature of the ablated material above the substrate temperature of 7000 K decreases below 1000 K at a distance below 4 cm from the substrate but that for oxygen is still high. This decrease shows the possibility of adsorption of oxygen on the substrate.

The graph of the temperature of the atoms in the shock wave vs. the pressure of the background oxygen near the substrate is shown in Fig. 12. As the background oxygen pressure is increased, its temperature within the thin layer increases, while that of vapor decreases rapidly. This indicates that the ablated materials transfer their kinetic and thermal energies to the oxygen molecules, and condense back to the target. This helps in the ionization of oxygen molecules for the production of oxygen atoms that are easily absorbed. Similarly, the energy of atoms near the substrate plotted against the background pressure is shown in Fig. 13. The energy of oxygen is high and that of vapor is low but decreases in the same way with an increment of pressure.

The background pressure can control the energy of the impinging particles from being incident onto the surface. The density of the atoms of the laser plume within the shock layer is of the order of 10¹⁶/cm², and the temperature is below 1000 K or 727 °C. This condition is favorable for the oxidation of the metal atoms, condensation, and the formation of the clusters. This temperature is equally suitable for the oxidation of MXene up to which all of its properties are preserved. These data are in good agreement with the previous literature.¹³ 

Reflection High Energy Electron diffraction (RHEED) characterizes crystalline surfaces with the help of a diffraction pattern with a spot as maximum intensity.²⁸ Cleaving or coating with passive oxides helps to get a clean surface,²⁹ which can be removed by annealing. The RHEED technique considers both reciprocal and real space observations.³⁰ The RHEED pattern at four different pressures [(1) p = 0.05 mbar, (2) p = 0.1 mbar, (3) p = 0.2 mbar, and (4) p = 0.3 mbar] for Mo₂TiC₂ and Mo₂Ti₂C₃ is studied in this work. The energy of the laser is 400 mJ with surface energy density F = 2 J cm⁻², target to substrate distance d_T–S = 4 cm, and pulse repetition rate ν = 2 Hz.

The pressure is adjusted to p = 0.05 mbar, and E_k is in the order of 10 eV. The stopping distance (average path before thermalization) for the plume is larger than the deposition distance, i.e., d_stop ≫ d_T–S. The strong damping in the oscillation (0, 0) indicates the transition to a multilayer system in both Mo₂TiC₂ and Mo₂Ti₂C₃ as shown in Figs. 14 and 15, respectively. In addition, the growth is so focused on one point that even adsorption is not completed on one unit cell. The deposition takes place on the top of the island. In this condition, aluminum is also deposited on the surface, resulting in a useless surface. However, the surfaces of both the MXenes look similar. The reconstruction of the pattern is due to the post nucleation process. At this pressure, the circular spots (0, 0) and (0, ±1) are distorted to rods, indicating the more disordered surface of both MXenes as shown in Figs. 16 and 17, respectively. The oscillating pattern gives the deposition rate. The kinetic energy of the ablated particles is so high that the surface uniformity and flatness are destroyed. Both the initial and final patterns of are different.

RHEED oscillatory behavior during adsorption in Mo₂TiC₂ under p = 0.1 mbar and Mo₂Ti₂C₃ under p = 0.1 mbar is shown in Figs. 18 and 19, respectively. RHEED patterns of the starting surface of Mo₂TiC₂ and the final surface of Mo₂TiC₂ are shown in Figs. 20(a) and 20(b), respectively. RHEED patterns of the starting surface of Mo₂Ti₂C₃ and the final surface of Mo₂Ti₂C₃ are shown in Figs. 21(a) and 21(b), respectively.

RHEED oscillatory behavior during adsorption in Mo₂TiC₂ under p = 0.2 mbar and Mo₂Ti₂C₃ under 0.2 mbar is shown in Figs. 22 and 23, respectively. RHEED patterns of the starting surface of Mo₂TiC₂ and the final surface of Mo₂TiC₂ are shown in Figs. 24(a) and 24(b), respectively. RHEED patterns of the starting surface of Mo₂Ti₂C₃ and the final surface of Mo₂Ti₂C₃ are shown in Figs. 25(a) and 25(b), respectively.

For the pressure of 0.01 mbar, there are three spots and reciprocal lattice rods seen on the zeroth order Laue circle with the respective point (0, 0) at the center and (0, −1) and (0, +1) on the left and right, respectively [Figs. 20(a) and 21(a)]. There are three sharp brighter and circular spots and little stretched rods in the final diffraction better than the initial one as shown in Figs. 20(b) and 21(b). This confirms the flatness and smoothness of the substrate surface. There is an intensity variation during growth at the (0, 0) spot, showing an oscillatory behavior as shown in Figs. 18 and 19 for Mo₂TiC₂ and Mo₂Ti₂C₃, respectively. The intensity continuously increases up to 800s (nearly 14 min), indicating the improvement in the smoothness and flatness of the surface compared with the initial one. The kinetic energy of the ablated species is calculated from the plume dynamic analysis and found to be in the order of a few eV. Here, d_stop ≈ d_T–S.

The (0, 0) spot intensity (at p = 0.2 mbar) is varied with adsorption time and better than that at p = 0.1 mbar. The (0, 0) RHEED intensity shows an oscillatory nature almost with the same amplitude, indicating the adsorption of good quality again as shown in Figs. 22 and 23. The damping indicates the ending of the adsorption process. The first four maxima indicate the quality adsorption of four monolayers (ML). The kinetic energy is in the order of 0.1 eV, and d_stop ≤ d_T–S. At this pressure, the ablated material does not reach the substrate rather it thrusts and ionizes the oxygen atoms. The respective initial and final RHEED patterns are shown in Figs. 24 and 25, respectively.

The pressure is changed to p = 0.3 mbar, and the estimated kinetic energy is in the order of 0.01 eV, which is not sufficient even to ionize the oxygen molecules into oxygen atoms. The plume stopping distance is much smaller than the deposition distance, i.e., d_stop ≪≪ d_{T −S}. The mobility of oxygen ions is very low. The rare oscillation at (0, 0) indicates no adsorption in both Mo₂TiC₂ and Mo₂Ti₂C₃ as shown in Figs. 26 and 27, respectively. At this pressure, the circular spots (0, 0) and (0, ±1) are distorted into rods, indicating the more disordered surface of both MXenes as shown in Figs. 28 and 29, respectively. The kinetic energy of the ablated particles is so small that there is a slight effect on the surface uniformity and flatness.

The kinetic energy of the species impinging onto the substrate (E_k), d_stop vs d_T–S, E_k vs E_A, and the nature of oscillation are summarized in Table I.³¹ 

It can be seen from this table that the kinetic energy of the ablated species has an inverse relation with the deposition pressure. The thermal energy obtained from the kinetic energy is given by E_th = k_BT and has a contribution to the mobility of the oxygen atoms to be adsorbed. In addition, it damages the surface with non-recoverable defects.^32–36 The pressures of 0.1 and 0.2 mbar for a pulse energy of 400 mJ with an energy density of 2 J cm⁻² are good for the adsorption of oxygen.

A new ablated plasma thrust method is successfully used for the ionization and adsorption of oxygen on the MXene substrate in the PLD chamber. It is found that the background pressure has a negative effect and the substrate temperature has a positive effect on plume expansion. The density profile of the background gas is highly affected by deposition temperature. Similarly, the plasma generated by longer wavelengths is more cylindrical than that generated by shorter ones due to the inverse bremsstrahlung process. The transfer of the kinetic energy of the confined plasma plume to thermal energy is very important for the ionization and activation of molecular oxygen. From the RHEED study, it is found that the ambient gas pressures p = 0.1 mbar and 0.2 mbar are appropriate for the adsorption process of the oxygen atoms to be adsorbed at the hollow sites of Mo₂TiC₂ and Mo₂Ti₂C₃. This might be the first approach in which we use ablated species only for ionizing and pressurizing the background gas onto the substrate for adsorption in the form of complex MXene oxide. The microstructural study shows close agreement with the RHEED measurement.

We thank Dr. S. N. Jha and his team for their support in obtaining experimental data at the RRCAT, Indore.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.