Role of $\pi \text{−}d$ hybridization in a 300-K organic-magnetic interface: Metal-free phthalocyanine single molecules on a bcc Fe(001) whisker

The realization of single molecular electronics is considered the next frontier to addressing and sustaining the storage needs of the future. In order to realize a single molecular device working at 300 K, two conditions must be satisfied: first, there must be no molecular diffusion, i.e., robust bonding between molecules and the contacting electrode, and second, stable electronic interface states. In this study, using a combination of 7-K and 300-K ultrahigh vacuum scanning tunneling microscopy/spectroscopy experiments and theoretical ab initio calculations, we investigated the adsorption of $\pi$-conjugated metal-free phthalocyanine (Pc) single molecules onto an Fe(001) whisker single crystal along with the resulting electronic interface structures. The Pc/Fe(001) system was found to prevent molecular diffusion even at 300 K, due to strong adsorption as well as the presence of a larger diffusion barrier than that of the Pc/Ag(001) system, in which molecules are known to diffuse at 300 K. The origin of such a robust bonding was studied by recovering the sample local density of states (LDOS) with the normalized $(dI/dV)/T$ curves, where the LDOS peaks are successfully explained by theoretical calculations.

Figure 1

(a) A picture of a furnace growing Fe whiskers from $\mathrm{FeC}{\mathrm{l}}_2$ powders with ${\mathrm{H}}_2$ gas, chemical vapor deposition (CVD) process. (b) A picture of grown Fe whiskers inside the $\mathrm{Si}{\mathrm{O}}_2$ tube. (c) Scanning electron microscopy (SEM) image of the single Fe whisker. (d) X-ray Laue image obtained from the whisker. (e) Magnetic domain image by our homebuilt SEM with polarization analysis (SEMPA). (f) Scanning tunneling microscopy (STM) topographic images of the Fe(001) whisker surface (215 × 108 nm). (g) An atomically resolved STM image of Fe(001) (1.1 × 1.1 nm).

Figure 2

(a) STM topographic image of the Fe(001) substrate covered by 0.1 ML of ${\mathrm{H}}_2\mathrm{Pc}$ molecules $(V_S=\text{–}1.0\mathrm{V},I=50\mathrm{pA},20\times 20\mathrm{nm})$, observed at room temperature. The single molecules are observed. White and black arrows indicate type A and B molecules adsorbing on the bcc Fe(001) with different orientations. Rarely, the molecule marked by the circle was observed: type C. The enlarged image [the inset in (a)] shows a protrusion at the core of the molecule, while there is no protrusion at the core for type A and B molecules. Adsorption probabilities are 56, 39, and 5%, for types A, B, and C, respectively. (b), (c) Enlarged STM topographic images and line profiles of types A and B molecules $(V_S=\text{–}1.5\mathrm{V},I=300\mathrm{pA},5\times 5\mathrm{nm})$.

Figure 3

STS measurements at 7 K and 300 K performed on the single ${\mathrm{H}}_2\mathrm{Pc}$ molecules (type A) adsorbed on the Fe(001) whisker. (a) $dI/dV$ curves $(V_s=\text{–}0.5\mathrm{V},I=500\mathrm{pA})$ were obtained at 7 K in UHV with tip no.1 on the ${\mathrm{H}}_2\mathrm{Pc}$ single molecule (black circles) and the Fe(001) substrate (gray circles). A dotted line denotes a fitted $T$ function. (b) $(dI/dV)/T$ curve (black circles) obtained from the $dI/dV$ curve in (a) with the fitted $T$. The $(dI/dV)/T$ curve was fitted with Gaussian curves. The fitted curve is shown as a black line. Obtained Gaussian peaks were used to determine LDOS peak energy positions labeled I (blue), II (red), III (green), and IV (pink). (c), (d) $(dI/dV)/T$ curve (black circles) obtained on the single ${\mathrm{H}}_2\mathrm{Pc}$ molecule at 300 K with a different tip (tip no. 2). The $(dI/dV)/T$ curves were fitted with Gaussian curves. The fitted curves are shown as black lines. Obtained Gaussian peaks were used to determine LDOS peak energy positions labeled I (blue), II (red), III (green), IV (pink), and V (gray), (c) obtained at the core and (d) obtained at the arms. Insets in (d) (5 × 5 nm) show a topographic image and a $dI/dV$ map at +0.9 V.

Figure 4

$(dI/dV)/T$ curve (black circles) measurements at 300 K performed on the type-B and -C single ${\mathrm{H}}_2\mathrm{Pc}$ molecules adsorbed on the Fe(001) whisker. The $(dI/dV)/T$ curves were fitted with Gaussian curves. The fitted curves are shown as black lines. Obtained Gaussian peaks were used to determine LDOS peak energy positions labeled I (blue), II (red), III (green), IV (pink), and V (gray). (a), (b) Obtained for the type-B molecule at the core and the arms inside the single molecule, respectively. Insets in (a) (5 × 5 nm) show a topographic image and a $dI/dV$ map at +0.9 V. (c), (d) Obtained for a type-C molecule at the core and the arms inside the single molecule, respectively. An inset in (c) (5 × 5 nm) shows a topographic image.

Figure 5

Calculated spin-resolved LDOS of (a), (b) a freestanding ${\mathrm{H}}_2\mathrm{Pc}$ molecule and (c), (d) a single ${\mathrm{H}}_2\mathrm{Pc}$ molecule (type A) adsorbed onto bcc Fe(001) substrate. The top and bottom sections show the majority and minority spin states, respectively. I, II, III, and IV denote the energy positions of experimentally determined peaks. (e) Spherical model of a ${\mathrm{H}}_2\mathrm{Pc}$ molecule on a square atomic lattice. H (blue sphere), C (gray sphere), and N (yellow sphere) form the constituent chemical species of the molecule. Four nitrogen and eight carbon atoms marked by the red circles are the main contributed atoms for the core. The 16 carbon atoms marked by the blue circles are representatives of the arms. (f), (g) Interface hybridization between ${\mathrm{H}}_2\mathrm{Pc}$ and Fe(001) in the minority (f) and the majority (g) spin states. In (f), energy positions of the Fe(001) $d_{\mathrm{xz}+\mathrm{yz}}$ surface states and the molecular HOMO/LUMO states are shown as red and black lines, respectively. Blue lines denote new hybridized bonding/antibonding states. I and IV denote experimentally obtained peak positions. In (g), there is no peak at the Fe(001) side. No hybridization occurs. (h) Three-dimensional views of minority and majority spin interface states: hybridized molecular $\pi$ and Fe(001) $d$ states.

Figure 6

(a) Calculated LDOS of a single ${\mathrm{H}}_2\mathrm{Pc}$ molecule adsorbed onto a fcc Ag(001) substrate. (b) Adsorption energy of a single ${\mathrm{H}}_2\mathrm{Pc}$ molecule onto Fe(001) (black dots) and Ag(001) (white dots). The $d$ axis represents the molecule-substrate distance. (c) Energy difference of the diffusion barrier from the fourfold hollow site position (H) for ${\mathrm{H}}_2\mathrm{Pc}$ molecules on Ag(001) (white dots) and Fe(001) (black dots). A top site (T), a bridge site along the [010] direction (B1), and a bridge site along the [100] direction (B2) are calculated. (d) Continuous STM images of a given area on which 0.1-ML ${\mathrm{H}}_2\mathrm{Pc}$ was adsorbed onto Fe(001) observed at 300 K $\left(V_s=\text{–}600\mathrm{mV},I=600\mathrm{pA},20\times 20\mathrm{nm}\right)$. The images were sequentially acquired at 5-min intervals. (e) STM images (20 × 9.4 nm) of this area at different tunneling resistances of $1\times {10}^9\mathrm{Ohms}(V_s=\text{–}600\mathrm{mV},I=600\mathrm{pA}),2\times {10}^8\mathrm{Ohms}(V_s=\text{–}600\mathrm{mV},I=3\mathrm{nA}),2\times {10}^7\mathrm{Ohms}(V_s=\text{–}600\mathrm{mV},I=30\mathrm{nA})$, and $3\times {10}^6\mathrm{Ohms}(V_{\mathrm{s}}=\text{–}100\mathrm{mV},I=30\mathrm{nA})$. Five single molecules (bright spots) did not move.