We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS 2 channel when the channel part is exposed to Cl 2 gas. The gas exposure and the electrical measurement of the MoS 2 FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS 2 FET can provide and understand the surface properties. With the Cl 2 gas exposure to the channel, the plot of the drain current versus the gate voltage (I d −V g curve) shifts monotonically toward the positive direction of V g , suggesting that the adsorbate acts as an electron acceptor. The I d −V g shifts are numerically estimated by measuring the onset of I d (threshold voltage, V th ) and the mobility as a function of the dosing amounts of the Cl 2 gas. The behaviors of both the V th shift and the mobility with the Cl 2 dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS 2 surface up to the monolayer coverage. The charge transfer makes the V th shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS 2 channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl 2 molecule. The monotonic shift of V th with the coverage suggests that the MoS 2 device sensor has a good sensitivity to detect 10 −3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.

We investigated the spin properties of the terbium phthalocyanine (TbPc) species adsorbed on the superconductor NbSe 2 surface using scanning tunneling microscopy and spectroscopy. TbPc 2 is a molecule in a class of single-molecule magnets (SMMs), and the use of superconductor electrodes attracts attention for the application to the devices using the spin degree of freedom. TbPc is a building block of TbPc 2 and can reveal the spin component’s behavior. In the experiment, TbPc species were placed on the surface of the superconductor NbSe 2 . We measured Yu–Shiba–Rusinov (YSR) states caused by the interaction between the superconducting state and magnetic impurity and inelastic tunneling spectroscopy (IETS) for the spin excitation, below 1 K. We also measured the Kondo state formed by the magnetic singlet formation. We detected the radical spin at the ligand position of the TbPc by the presence of the Kondo peak and demonstrated that the radical spin forms the YSR feature. In addition, the exchange interaction energy ( E ex ) between the spins of the radical ligand (Pc) and the center 4f metal atom (Tb 3+ ) is determined by using the IETS technique. E ex is a critical parameter that determines the blocking temperature, below which the sample behaves as an SMM. IETS results show that the statistical distribution of E ex has peaked at 1.3, 1.6, and 1.9 meV. The energy range is comparable to the recent theoretical calculation result. In addition, we show that the energy variation is correlated with the bonding configuration of TbPc.

We demonstrate that a nonmagnetic molecule is changed into magnetic when its internal structure is rotated by forming an ordered film on a metal surface. We used a double-decker complex, bis­(phthalocyaninato)­cerium (CePc2), where the valence of the Ce atom changes between +3 and +4 with a small energy barrier. CePc2 is nonmagnetic in bulk and becomes magnetic in the monolayer film on Au(111). The intra-molecular structure and the magnetic property are revealed using scanning tunneling microscopy/spectroscopy. We attribute the mechanism to a change of the rotational angle (θ) between the upper and lower Pc ligands, driven by the steric repulsive force originating from forming an almost square lattice on Au(111). The θ changes from θ = 45° of the bulk value into θ = 0° for the film. GGA + U calculation indicates that the change from θ = 45° to θ = 0° causes the Ce valence change from the nonmagnetic +4 state into the magnetic +3 state.