A graphene-based hybrid material with quantum bits prepared by the double Langmuir–Schaefer method

The scalability and stability of molecular qubits deposited on surfaces is a crucial step for incorporating them into upcoming electronic devices. Herein, we report on the preparation and characterisation of a molecular quantum bit, copper(ii)dibenzoylmethane [Cu(dbm)2], deposited by a modified Langmuir–Schaefer (LS) technique onto a graphene-based substrate. A double LS deposition was used for the preparation of a few-layer-graphene (FLG) on a Si/SiO2 substrate with subsequent deposition of the molecules. Magnetic properties were probed by high-frequency electron spin resonance (HF-ESR) spectroscopy and found maintained after deposition. Additional spectroscopic and imaging techniques, such as Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were performed to characterise the deposited sample. Our approach demonstrated the possibility to utilise a controlled wet-chemistry protocol to prepare an array of potential quantum bits on a disordered graphene-based substrate. The deployed spectroscopic techniques showed unambiguously the robustness of our studied system with a potential to fabricate large-scale, intact, and stable quantum bits.

2) Langmuir -Schaefer deposition setup scheme.Additionally, the four-point-probe resistance measurements were performed on bare Si/SiO2, FLG, and also a CVD graphene sample and revealed 10 6 times higher conductivity of FLG compared to Si/SiO2 and around 50 times higher conductivity for CVD graphene compared to FLG.Therefore, it is possible to tune conductivity based on the used substrate. 1) Langmuir -Schaefer deposition setup scheme.
Figure S2 shows experimental setup for modified Langmuir-Schaefer deposition which started by injecting the FLG suspension onto a subphase (deionised water in this case) surface (1).The movable barriers (2) were slowly closed and reduced the surface area of a trough top (1).The process of a layer formation was observed by an optical microscope (3) with a visual output (4) to a monitor, and a Wilhelmy plate* made out of paper for monitoring the surface pressure (5).The (6) is the Si/SiO2 substrate on which the FLG deposition took place.Finally, the suction pump (7), by which the water was pumped out, and thus the water level was lowered and the deposition was done.The term modified stands for the deposition carried out by elevating and tilting the substrate.This was done by placing metal nuts of different sizes underneath the substrates.The force acting on this plate is sum of three force contributions; the gravity  G and the surface tension  ST , both are acting downwards, whereas the buoyancy  B acting on the plate is acting upwards.This can be expressed by following equations: for a rectangular plate of dimensions  p ,  p ,   , material density  p , and perimeter P immersed to a depth h in a liquid of density  l the net force is given by: where  is the surface tension of the liquid,  is the contact angle on the solid plate and  is the gravitational constant.By this approach it is possible to measure surface pressure by measuring the change of force F acting on a plate with and without a molecular film at the surface.During a deposition, Wilhelmy plate is completely wetted after a while, that means  = 0 and cos 0 = 1.The surface pressure can be subsequently obtained from following equations: giving the final relation for surface pressure connecting the change in force and the change in surface tension The sensitivity is further increased by using a very thin plate, so that  p ≪  p , and giving the following equation The force is then determined by measuring the changes in the mass of the plate, which is directly coupled to a sensitive microbalance. 2igure S4 shows experimental setup for the second modified Langmuir-Schaefer deposition which started by injecting [Cu(dbm)2] onto a subphase (also deionised water) surface (1).The movable barriers (2) were slowly closed and reduced the surface area of a trough top (1).The Wilhelmy plate made out of paper (3) was used to monitor the surface pressure.The (4) represents the graphene-covered Si/SiO2 substrate from previous FLG deposition.And finally, the suction pump (5), by which the water was pumped out, and thus the water level was lowered and the deposition done.The term modified stands for the deposition carried out by elevating and tilting the substrate by a flat metal washer placed under the graphene-covered substrate.By this deposition, the 5 mM solution of [Cu(dbm)2] in chloroform was deposited onto FLG-covered substrate in a controlled wet-chemistry manner.This multiple modified Langmuir-Schaefer deposition is scalable, reproducible, and presents a feasible deposition route.
3) Isotherms obtained during blank deposition of [Cu(dbm)2] without substrates and the actual deposition.
When the molecular layer was compressed on the water surface, it underwent phase transformations.These changes were observed by monitoring the surface pressure Π as a function of the area occupied by the film A. Figure S4 shows this diagram and is unique for every compound and gives information about the formation of the layer.It is common, in such a plot, to divide the film area A by the total number of molecules N on the water surface to obtain the area per molecule as follows: , where A is the actual trough top area, NA is Avogadro constant, c is the molar concentration of the solution, and V is the volume injected onto a water subphase.The molar concentration for both 'Finding' and 'Deposition' of [Cu(dbm)2] was 5 mM.The volume VFINDING = 750 μL and VDEPOSITION = 1400 μL.The isotherm was a very first characteristic of the compound regarding the deposition step.During the 'Finding' deposition, the isotherm was estimated to be at Π = 25 mN/m (i.e.where the curve starts to saturate).This pressure was then applied during the actual deposition onto substrates.The process of finding a correct isotherm is crucial for the formation of the ideal coverage.This was, however, an uneasy task for metal compounds.Surface pressure is given as follows:  =  0 − , where  0 is the surface tension of the pure liquid and  is the surface tension of the film-covered surface.Figure S7 illustrates the main setup components of HF-ESR spectrometer located at the University of Stuttgart, Germany.The tuneable microwave source (1) provided radiation of variable frequency: ν = 82 − 1100 GHz, which was propagated by a quasi-optics (2).The higher frequencies were accessed by amplifying and multiplying the microwave base frequency: ν = 8 − 12 GHz.The variable temperature insert (3), T = 1.8 − 300 K, was put in a tunable superconducting magnet (4) capable of magnetic field up to ±17 T. The microwave detection was provided by an InSb bolometer (5) cooled by the liquid nitrogen and helium in order to increase its sensitivity.5) Table S8.Raman comparison of peaks found with their assignment.

Figure S1 .
Figure S1.Characterisation of FLG layer for its thickness and uniformity by a) optical microscopy, b) AFM image, c) SEM image, and d) HR-TEM image.

Figure S3 .
Figure S3.Wilhelmy plate scheme.The perspective view and the side view.

Figure S5 .
Figure S5.Diagram a) and b) show isotherms (a curve on a Π-A isotherm diagram at the constant room temperature) for [Cu(dbm)2] obtained during the blank deposition and actual deposition onto substrates, respectively.

Figure S6 .
Figure S6.Characterisation of deposited [Cu(dbm)2] on FLG.a) SEM image obtained at 5 kV and equipped with secondary electrons detector, b) AFM topography image c) extracted 3D morphology image.

Figure S7 .
Figure S7.The HF-ESR spectrometer.a) Indication of setup components, b) variable temperature insert (VTI), c) detail of sample-rod head with modulation coil, at which the alternating current (AC) with known noise frequency was applied and formed a lock-in amplification of signal (the first derivative of absorption signal was detected), d) sample insert with support, and e) ∅5 mm sample in the form of a pressed pellet (for a bulk powder measurements) or a thin film (deposited [Cu(dbm)2] on FLG-covered substrate).