PHYSICAL REVIEW B 94, 195437 (2016)

Role of π-d hybridization in a 300-K organic-magnetic interface: Metal-free phthalocyanine single
molecules on a bcc Fe(001) whisker
T. K. Yamada,1,2,* Y. Yamagishi,1 S. Nakashima,1 Y. Kitaoka,3 and K. Nakamura4
1

Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan2
Molecular Chirality Research Center, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
3
National Institute of Advanced Industrial Science and Technology (AIST), Spintronics Research Center, 1-1-1 Umezono,
Tsukuba, Ibaraki 305-8568, Japan
4
Department of Physics Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu City 514-8507, Mie, Japan(Received 3 February 2016; revised manuscript received 31 October 2016; published 28 November 2016)
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 thecontacting 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 π -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 presenceof 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.
DOI: 10.1103/PhysRevB.94.195437
I. INTRODUCTION

The realization of single-molecule devices is considered thenext frontier to addressing and sustaining the storage needsof the future. In order to realize a single-molecule junction
working at 300 K, one condition must be satisfied: there
must be robust bonding between molecules and the contacting
electrode. The drastic increase in the use of computers
and the internet has led to an increasing demand for highcapacity storage devices. Within current limitations in termsof resources and energy, nanotechnology and nanomaterials,
which have aided in significantly reducing device sizes, havebeen considered as a key solution to building devices that canaddress the memory needs of the future. In this backdrop,
researchers have focused on nanomaterials such as carbon
nanotubes, graphene, and molecular films to realize compact
and flexible electronic devices [1–8]. Single molecules withsizes less than 1 nm have attracted attention as potential
nanomaterials for future electronic devices [9–38].
Single organic molecules adsorbed onto various electrodes
have been studied using both scanning tunneling microscopy(STM) as well as STM break-junction approaches. Insulating
electrodes such as MgO, NaCl, and CuN [9–11] are powerfultemplates to visualize intrinsic molecular highest occupied andlowest unoccupied molecular orbital (HOMO and LUMO, respectively) states. Electronic interactions determine the bonding strength between π -conjugated single organic moleculesand substrate electrodes, i.e., the insulating electrode exhibits
negligible electronic interaction with the substrate.
Metallic substrates, mainly noble metals such as Au, Ag,
Cu, and Pt, are also frequently used to realize phenomena
such as Kondo resonance [12–23]. It is well known that*

Corresponding author: toyoyamada@faculty.chiba-u.jp

2469-9950/2016/94(19)/195437(9)π -conjugated single molecules adsorbed onto noble metalelectrodes such as fcc Au, Ag, Cu, or Pt diffuse and forma two-dimensional ordered film at 300 K [20,22–25]. Because
the molecules move across the noble metal electrode surface,in break-junction measurements, the single molecule can “fit”
between the two electrodes [32–38]. Noble metal surfaces, inwhich mainly s or p states are dominant near the Fermi energy(EF ), interact weakly with the molecule, essentially throughthe process of physisorption.
With metal-free single organic molecules supported by
3d magnetic metals such as Fe, Co, Cr, and Mn, giant
magnetoresistance (GMR) junctions have been successfully
realized in a spin-polarized STM; the GMR value thusachieved was +60% for [Co(111)/H2 Pc/Co(111)], −50% for[Mn(001)/H2 Pc/Fe(110)], and −30% for [Fe/C60 /Cr] single
molecular junctions [14–16]. The interfaces between organic
molecules and 3d magnets appear to be spin-polarized even
without the presence of magnetic ions in molecules due to hybridization between the d and π states, and thus, the molecules
are likely chemisorbed; consequently, the distance between thesubstrate and the molecule is less than that in the physisorption
case due to the strong nature of chemisorbed bonds.
In this study, metal-free phthalocyanine (H2 Pc) single
molecules are investigated as one of the ideal metal-free
π -conjugated molecules by means of ultrahigh vacuum (UHV)
scanning tunneling microscopy and spectroscopy (STM/STS)at 7 K and room temperature (RT). We focus on using a bcc
Fe(001)-whisker single crystal as the substrate electrode since
the procedure to obtain a uniform surface, distinct electronic
spin structures, and clear magnetic domain patterns for thissingle crystal has been well studied [39–41]. Further, whiskers
prepared via the chemical vapor deposition method containextremely low concentrations of impurities when comparedwith those in commercial Fe single crystals. 0.1-ML H2 Pc
molecules were adsorbed on the Fe(001) whisker at 300 K.

195437-1©2016 American Physical Society

T. K. YAMADA et al.

PHYSICAL REVIEW B 94, 195437 (2016)

Different types of adsorption sites and new local density ofstate (LDOS) peaks were confirmed. Ab initio calculations
identified the origin of the new interface state peaks, i.e.,hybridization of molecular π and Fe(001) d states. Different
LDOS inside single H2 Pc molecules are confirmed experimentally and theoretically. With a comparison of H2 Pc/Ag(001),
where there is no hybridization of π -d states, H2 Pc molecules
on Fe(001) were found to have a more than 3 times higher
diffusion barrier and adsorption energy. Experimentally, H2 Pc
molecules on Fe(001) did not diffuse and immobilize by STM
manipulation, indicating worse knowledge to realize 300 K
single molecular devices.

(b)

(a)

Fe whiskers
FeCl2

H2

SiO2 tube

bcc-Fe(001)

(d)

(c)
<100>

II. METHODS AND EXPERIMENTS
A. STM and spectroscopy measurements(001)

STM experiments were performed at 7 K and 300 K
in ultrahigh vacuum (UHV, base pressure < 8 × 10−9 Pa).
One of the most effective techniques for precisely measuring
the LDOS of 1-nm-size molecules is tunneling spectroscopy
with an STM setup, which is known as scanning tunnelingspectroscopy [24]. STS measurements were performed by
opening the feedback loop of the tunneling current, i.e.,
fixing the tip-sample separation (z). The tunneling current
was measured at each pixel by varying the sample voltage
to obtain the I(V) curves, where zero voltage corresponds
to the Fermi energy. The differential conductivity (dI/dV)
curves were obtained by numerical differentiation of the I(V)curves. The LDOS was obtained by the normalization of
dI/dV curves using the T background, i.e., (dI/dV)/T below
the Fermi energy indicates occupied LDOS and (dI/dV)/T
above the Fermi energy indicates unoccupied LDOS, where
T denotes √the fitted tunneling probability
functions: T =√a exp(−2z φ − eV /2) + b exp(−2z φ + eV /2), where aand b denote fitting parameters, φ the averaged barrier height
between tip and sample, and z the tip-sample separation[24,42–45]. In this study, we used φ = 4.5 eV and z =10 Å, but we also tested the normalization with different φ(4.0−5.5 eV) and z (5−11 Å), while we always succeeded
to recover the peaks at the same energy positions within ±20 meV (see also Ref. [24]).
B. Sample and tip preparations

STM tungsten (W) tips were electrochemically etched fromW wires (φ = 0.3 mm, purity 99.9%) in air using aqueous
KOH. The tip was rinsed with hot water and acetone andsubsequently transferred into the load-lock chamber of our
STM setup. The apex of the W tip was annealed to obtain an
impurity-free apex under UHV conditions [46].
Fe-whisker single crystals were grown by using the chemical vapor deposition (CVD) technique to obtain high-purity Fe
samples [39–41]. As shown in Fig. 1(a), the FeCl2 powder was
set in the quartz tube inside the furnace. By flowing H2 gas into
the tube at 750 °C, Fe whiskers were grown inside the tube [seeFig. 1(b)]. The scanning electron microscopy (SEM) image in
Fig. 1(c) shows that one whisker has a rectangular shape with a
10–50 mm length and a 0.1–1-mm side. The x-ray Laue image
in Fig. 1(d) confirmed that the our Fe whiskers have bcc (001)surfaces at the side planes. Magnetic imaging by SEM with

0.1mm

1mm

<100>

(e)
100 nm

(f)

(g)

[100]
[010]
FIG. 1. (a) A picture of a furnace growing Fe whiskers from FeCl2
powders with H2 gas, chemical vapor deposition (CVD) process. (b)A picture of grown Fe whiskers inside the SiO2 tube. (c) Scanningelectron microscopy (SEM) image of the single Fe whisker. (d) Xray 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 theFe(001) whisker surface (215 × 108 nm). (g) An atomically resolved
STM image of Fe(001) (1.1 × 1.1 nm).

polarization analysis (SEMPA) showed magnetic domains of
the Fe whisker. The typical size is 0.5 × 1.0 mm, and theeasy axis is in the 100 direction (long axis). The whisker
that was introduced from air into the UHV chamber was firstcleaned by Ar+ sputtering and flattened by annealing up to
870 K nearly two dozen times. Before STM measurements, thewhisker was sputtered at 870 K for 30 min. The annealing was
stopped 5 min after stopping the sputtering, which produced aclean and atomically flat Fe(001) surface. Figure 1(f) shows a
topographic STM image of the Fe(001) whisker single crystal
at room temperature in ultrahigh vacuum. Atomic terraces
larger than 100 nm are observed. The atomically resolved
STM image in Fig. 1(g) indicates the presence of bcc (001)symmetry.
Commercial metal-free π -conjugated phthalocyanine(H2 Pc) powder (Alfa Aesar, purity 95%) was purified bysublimation at 653 K and recrystallization at 473 K undera pressure of 10−3 Pa (yield 30%). NMR and infrared spectraof the samples were used to confirm the absence of impurities
after purification. The clean H2 Pc powder was placed in a

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ROLE OF π -d HYBRIDIZATION IN A 300-K ORGANIC- . . .

PHYSICAL REVIEW B 94, 195437 (2016)

crucible, which was set in a molecular chamber and heated to550 K. H2 Pc molecules were deposited on the Fe(001) whisker
in a UHV preparation chamber and subsequently transferred
to an analytical STM chamber under UHV.
C. Ab i ni t i o calculations

Calculations were carried out based on the generalized
gradient approximation (GGA) [47] by using the full-potential
linearized augmented plane-wave (FLAPW) method withsingle-slab geometry [48–50]. LAPW basis sets with cutoffs
of |k + G|  3.6 a.u.-1 and muffin-tin (MT) sphere radii of
2.2 a.u. for Ag and Fe, 1.2 a.u. for N and C, and 0.8 a.u. for H
were used; lattice harmonics with angular momenta up to l = 8
for Ag and Fe, 6 for N and C, and 4 for H were employed to
expand the charge and spin densities. As a model, we adopted asingle H2 Pc molecule on a three-layer Ag(001) [and Fe(001)]
slab with four different adsorption sites, wherein the centerof the molecule locates at the hollow, top (T), and saddle(B1 and B2) sites on the substrate surface. The in-plane bulk
lattice constants of Ag and Fe were assumed, and the atomic
positions of H2 Pc and the substrate up to the second layer fromthe surface were fully optimized by atomic force calculations,while the layer–layer distance on the opposite side was fixed
to the corresponding bulk values.
III. RESULTS AND DISCUSSION

We deposited about 0.1-ML H2 Pc molecules on theFe(001)-whisker substrate at 300 K in UHV and subsequently
loaded the sample into the STM. STM/STS measurements
were performed at 7 K and 300 K. H2 Pc molecules wereobserved like a four-leaf clover on the Fe(001) as shownin Fig. 2(a), i.e., H2 Pc single molecules did not “stand” but“lay” on the Fe(001) as hybridized molecular π and Fe(001)
H2Pc / Fe(001)-p(1x1) @ 300K

(a)

A(
B(
C(

(b) type A): 56%): 39%): 5%(c) type B

[100]

[010]
[010]
o

θ =23+2
0.3

0.2
0.1
0.0
0 1 2 3 4 5x [nm]z [nm]

type Cz [nm]

0.3

0.2
0.1
0.0
0 1 2 3 4 5x [nm]

FIG. 2. (a) STM topographic image of the Fe(001) substrate covered by 0.1 ML of H2 Pc molecules (VS = –1.0 V, I = 50 pA, 20 ×
20 nm), observed at room temperature. The single molecules areobserved. 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 (VS = –1.5 V, I =300 pA, 5 × 5 nm).

d states. Each single molecule did not adsorb randomly. They
follow to adsorb in particular directions. We confirmed threeadsorption types: type A marked by black arrows, B marked
by white arrows, and C marked by the circle. Of the adsorbed
molecules on Fe(001), 56%, 39%, and 5% are type A, B, andC, respectively, suggesting that the type-A molecules are themost energetically stable.
An enlarged image of the type-A molecules is shown in
Fig. 2(b). Single H2 Pc molecules were observed as an assembleof four bright protrusions. Its line profile (at Vs = –1.5 V)
shows that the molecule has about 200 pm height on Fe(001)and the center is about 50 pm lower. The four bright protrusions
are likely four side groups of the H2 Pc. We call the center the“core” and the side groups as “arms.” The type-A molecule
adsorbed parallel to [010].
The type-B molecules could be the second most stable
adsorption site. Its enlarged image is shown in Fig. 2(c).
Similar to the type-A molecule, four bright protrusions wereobserved at about 200 pm height. The type-B molecules adsorb
about 23 ± 2 deg out of [010].
Rarely, we observed the type-C molecule. The inset image
in Fig. 2(a) shows an enlarged image. Different from type-A
and -B molecules, type-C molecule had extra protrusion at
the core, i.e., something might exist at the center of the H2 Pc
molecule. The type-C molecule tilted about 45 ± 2 deg from[010]. By a chance, there is possibility to enter at the core.
Possible candidates are an Fe atom from the substrate, a W
atom from the tip, and impurity atoms such as C or O. Thus,
the type-C molecules might not be H2 Pc.
We investigated LDOS of the type-A, -B, and -C single
molecules observed in Fig. 2 by using STS. Figure 3 shows
STS results obtained on the type-A molecule. Figure 3(a)shows dI/dV curves obtained at the core (black circles) andthe Fe(001) substrate (gray circles) at 7 K. On the substrate,
the sharp peak at +0.17 eV above the Fermi energy (i.e., theminority d states) proves that the observed surface is the bcc
Fe(001) substrate [39,41].
Compared to the Fe(001), the dI/dV curve at the coreshows peaks around –0.3 eV and +0.2 eV, and also ashoulder around +0.8 eV. These buried LDOS peaks are
successfully recovered in the normalized (dI/dV)/T curve by
its fitted T function (dotted line). Figure 3(b) shows theobtained (dI/dV)/T curve (black circles). LDOS peaks were
investigated by fitting with Gaussian functions. The fitting
curve is shown as a black line. Obtained Gaussian peaks
are named I, II, III, and IV, having energy positions ofpeak I −0.22 ± 0.16 eV, peak II +0.10 ± 0.05 eV, peak
III +0.21 ± 0.11 eV, and peak IV +0.53 ± 0.23 eV (seeTable I).
Next, we performed STS measurements at 300 K, where
LDOS peaks are expected to be broadened due to thermal
fluctuation. Figure 3(c) shows a (dI/dV)/T curve (black circles)
obtained at 300 K with a different tip (tip no. 2). The blackline shows an obtained fitting curve by using five Gaussianfunctions: peak I −0.29 ± 0.18 eV, peak II +0.14 ± 0.08 eV,
peak III +0.29 ± 0.12 eV, peak IV +0.52 ± 0.18 eV, and peak
V −0.84 ± 0.26 eV. Peaks I–IV exist around the same energy
positions as the peaks obtained at 7 K. On the other hand, peak
V around −0.85 eV was observed only with tip no. 2; therefore
we identify that peak V is a tip state (see Table I).

195437-3

T. K. YAMADA et al.

PHYSICAL REVIEW B 94, 195437 (2016)

Type A

T
-0.5

0.0
0.5
E - E F [eV]

1.0(c) @300K

III
V

-1.0

I

II
IV

-0.5

0.0
0.5
E - E F [eV]

-1.0

1.0

-0.5

IV

III

I

(dI/dV)/T ~ LDOS [arb. unit](dI/dV)/T ~ LDOS [arb. unit]

0.5
-1.0

II

0.0
0.5
E - E F [eV]

1.0

dI/dV map@+0.9 V

III
II

V

IV

I
-1.0

-0.5

@+0.9V

V

-0.5

IV

II

I

-1.0

III

0.0
0.5
E - E F [eV]1.0(b) Arms

V

II

III
IV

I

-1.0

-0.5

0.0
0.5
E - E F [eV]1.0Type C(d) @300K, arms
Topo.

(dI/dV)/T ~ LDOS [arb. unit]

1.5(a) Core
Topo. dI/dV map

0.0
0.5
E - E F [eV](c) Core(dI/dV)/T ~ LDOS [arb. unit]

dI/dV [nA/V]

Fe(001)(dI/dV)/T ~ LDOS [arb. unit]

H2Pc

2.5

(b) @7K(dI/dV)/T ~ LDOS [arb. unit](a) @7K(dI/dV)/T ~ LDOS [arb. unit]

Type B

Topo.

II

III IV

V
I

1.0

-1.0

FIG. 3. STS measurements at 7 K and 300 K performed on the
single H2 Pc molecules (type A) adsorbed on the Fe(001) whisker.
(a) dI/dV curves (Vs = –0.5 V, I = 500 pA) were obtained at 7 K in
UHV with tip no.1 on the H2 Pc single molecule (black circles) andthe 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 H2 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 × 5nm) show a topographic image and a dI/dV map at +0.9 V.

A topographic image and a dI/dV map at +0.9 V of thetype-A molecule are shown in Fig. 3(d). In the dI/dV map,the arms appear darker and the core appears brighter, showing
different LDOS exist inside the single molecule. A (dI/dV)/T
curve obtained at the arms is shown in Fig. 3(d): peak I –0.30 ± 0.15 eV, peak II +0.18 ± 0.09 eV, peak III +0.32 ±

-0.5

0.0
0.5
E - E F [eV]

1.0(d) Arms

II III
V

-1.0

IV

I
-0.5

0.0
0.5
E - E F [eV]1.0

FIG. 4. (dI/dV)/T curve (black circles) measurements at 300 K
performed on the type-B and -C single H2 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 typeC molecule at the core and the arms inside the single molecule,respectively. An inset in (c) (5 × 5 nm) shows a topographic image.

0.13 eV, and peak IV +0.56 ± 0.25 eV. The core and the arms
have the LDOS peaks at the same energy positions, only the
amplitude is different. Thus, we experimentally confirmed thatthe freestanding HOMO/LUMO are significantly influenced
by the contact of the molecule with Fe(001) via chemisorption.
In the same way, (dI/dV)/T curves obtained at the type-B
molecule are shown in Figs. 4(a) and 4(b). The dI/dV map of
the type-B molecule in (a) showed the arms appear darker andthe core appears brighter, similar to type A, indicating that thecore has higher LDOS amplitude. The obtained peak energy

TABLE I. Peak energy positions obtained by Gaussian fittings to the (dI/dV)/T curves experimentally obtained on the core and the arms ofthe types A, B, and C single H2 Pc molecules on Fe(001). Error bars denote a half-width of half maximum of each Gaussian peak.
Type

Area

Temperature [K]Tip

Peak I [eV]Peak II [eV]Peak III [eV]Peak IV [eV]Peak V [eV]

A
A
A
B
B
C
C

Core
Core
Arms
Core
Arms
Core
Arms

7
300
300
300
300
300
300

No. 1
No. 2
No. 2
No. 2
No. 2
No. 2
No. 2

–0.22 ± 0.16
–0.29 ± 0.18
–0.30 ± 0.15
–0.31 ± 0.16
–0.29 ± 0.15
–0.31 ± 0.16
–0.28 ± 0.14

+0.10 ± 0.05
+0.14 ± 0.08
+0.18 ± 0.09
+0.18 ± 0.07
+0.19 ± 0.10
+0.25 ± 0.12
+0.23 ± 0.12

+0.21 ± 0.11
+0.29 ± 0.12
+0.32 ± 0.13
+0.31 ± 0.14
+0.34 ± 0.15
+0.45 ± 0.08
+0.45 ± 0.19

+0.53 ± 0.23
+0.52 ± 0.18
+0.56 ± 0.25
+0.58 ± 0.37
+0.58 ± 0.27
+0.64 ± 0.24
+0.79 ± 0.30

None–0.84 ± 0.26
–0.86 ± 0.25
–0.85 ± 0.28
–0.86 ± 0.28
–0.86 ± 0.24
–0.88 ± 0.30

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ROLE OF π -d HYBRIDIZATION IN A 300-K ORGANIC- . . .

PHYSICAL REVIEW B 94, 195437 (2016)

positions are peak I −0.31 ± 0.16 eV, peak II +0.18 ± 0.07 eV,peak III +0.31 ± 0.14 eV, and +0.58 ± 0.37 eV for the core,and peak I −0.29 ± 0.15 eV, peak II +0.19 ± 0.10 eV, peak
III +0.34 ± 0.15 eV, and peak IV +0.58 ± 0.27 eV for thearms (see Table I). Compared with type A, all the peak energy
positions are identical. It is clear that there is no big differencein the LDOS between the type-A and type-B molecules.

(a) Core of free-standing H2 Pc(c) Core of H 2Pc/Fe(001)

Maj.

Maj.

Finally, we performed spectroscopy measurements for the
type-C molecule. Results are shown in Figs. 4(c) and 4(d). The
core likely includes an extra atom. Obtained peak positions arepeak I −0.31 ± 0.16 eV, peak II +0.25 ± 0.12 eV, peak III+0.45 ± 0.08 eV, and peak IV +0.64 ± 0.24 eV for the core,and peak I −0.28 ± 0.14 eV, peak II +0.23 ± 0.12 eV, peak III+0.45 ± 0.19 eV, and peak IV +0.79 ± 0.30 eV for the arms

I

Min.

LUMO

HOMO

Min.

II
III
IV

(d) Arms of H2 Pc/Fe(001)
II
III

Maj.

Maj.

(b) Arms of free-standing H2 Pc

-1

(e)

0 +1
E [eV]

Min.

Min.

-2

LUMO

HOMO

-3

+2

+3

I

-3

-2

Minority spin

(f)

-1
0 +1
E - EF [eV]

IV

]

00

[1

I

II II
III

/802

/802

()

I

(h)

+3

9/

()

)H 

+2

Majority spin

(g)

9/

6XUIDFHVWDWHV
[]\]

IV

+3F
)H 

II III

+202
+3F

+202
)H 

+3F)H 

+3F

IV
Pc

Maj.

Fe(001)Pc

Min.

Fe(001)
FIG. 5. Calculated spin-resolved LDOS of (a), (b) a freestanding H2 Pc molecule and (c), (d) a single H2 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 theenergy positions of experimentally determined peaks. (e) Spherical model of a H2 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 H2 Pc and Fe(001) in the minority (f) and the majority (g) spin states. In (f), energy positions of theFe(001) dxz+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 π and Fe(001) d states.
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T. K. YAMADA et al.

PHYSICAL REVIEW B 94, 195437 (2016)(d) Non-diffusion at 300 K
0 min.
5 min.

(a)

LUMO

HOMO

LDOS

H2Pc/Ag(001), core0
E - EF [eV]10(b)Pc/Ag(001)0-50Pc/Fe(001)d [pm]+4

1

Pc/Fe(001)

ΔE site [eV]Ads. energy [eV]

LUMO

-4

15 min.

10 min.

HOMO

LDOS

H2Pc/Ag(001), arms

500(c) (e) Immobilization at 300 K

Pc/Ag(001)0Tunnel registance [ ]=H T B1 B2

1x10

9

2x10

8

7

2x10

3x10

6

FIG. 6. (a) Calculated LDOS of a single H2 Pc molecule adsorbed onto a fcc Ag(001) substrate. (b) Adsorption energy of a single H2 Pc
molecule onto Fe(001) (black dots) and Ag(001) (white dots). The d axis represents the molecule-substrate distance. (c) Energy differenceof the diffusion barrier from the fourfold hollow site position (H) for H2 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 H2 Pc was adsorbed onto Fe(001) observed at 300 K (Vs = –600 mV, I = 600 pA, 20 × 20 nm).
The images were sequentially acquired at 5-min intervals. (e) STM images (20 × 9.4 nm) of this area at different tunneling resistancesof 1 × 109 Ohms(Vs = –600 mV, I = 600 pA), 2 × 108 Ohms(Vs = –600 mV, I = 3 nA), 2 × 107 Ohms(Vs = –600 mV,I = 30 nA), and 3 ×
106 Ohms(Vs = –100 mV, I = 30 nA). Five single molecules (bright spots) did not move.

(see Table I). Peak I did not change so much from types A andB, while for peaks II, III, and IV, positions shifted clearly. For
example, peak III shifted to +0.45 eV from around +0.3 eV
for types A and B. The (dI/dV)/T curve suggested that type C
could not be H2 Pc.
First-principles calculations were performed to investigate
the experimentally found four peaks (I, II, III, and IV) in(dI/dV)/T curves in Fig. 3. Figure 5 shows the calculation results of the energetically stable type-A molecule. Figures 5(a)and 5(b) show a calculated LDOS of a freestanding H2 Pc
molecule at the core (a) and the arms (b). HOMO-LUMO
states are observed with a gap of about 1.5 eV. Upper and lower
panels show majority and minority spin states, respectively.
The freestanding H2 Pc is not spin polarized.
Majority and minority spin states of the H2 Pc on Fe(001)were calculated as shown in Figs. 5(c) and 5(d) with theenergetically stable configuration in Fig. 5(e) based on
experimentally observed configuration in Fig. 2. Adsorptionenergy at the fourfold hollow site was energetically the moststable [Fig. 6(c)]. In Fig. 5(e), yellow, gray, and blue spheres
denote nitrogen, carbon, and hydrogen atoms, respectively.
The atoms marked by the red circles belong to the core, andthe atoms marked by the blue circles belong to the arms.

Figures 5(c) and 5(d) show LDOS at the core and the arms,
respectively.
Between majority and minority spin states, a significant
difference is observed from −2 eV to +2 eV, i.e., it isspin-polarized. This is not surprising, although so far no reportof H2 Pc on Fe(001), phthalocyanine, or porphyrin molecules
on Mn(001), Fe(110), and Co(0001) have been reported to bespin-polarized when they contact with 3d magnetic substrates.
(So far all reported experiments were performed at low
temperatures.)
In Fig. 3, we experimentally found four peaks around the
Fermi energy. In the calculated LDOS in Figs. 5(c) and 5(d),we observed three peaks around the Fermi energy. Comparing
Figs. 3, 5(c), and 5(d) shows that the origin of peaks II and
III is the majority spin states (yellow area), and the peak-I and-IV peaks mainly consist of the minority spin states. Similar
to the core, at the arms, the majority spin peak is observed atpeaks II and III, and minority spin peaks are observed at peaks
I and IV, while the peak amplitude is relatively smaller thanthe core.
Figures 5(f) and 5(g) show how the non-spin-polarizedstates of freestanding H2 Pc in Figs. 5(a) and 5(b) change
to spin-polarized states when H2 Pc contacts with Fe(001).

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PHYSICAL REVIEW B 94, 195437 (2016)

The longitudinal axis and horizontal axis denote energy andposition, respectively. The molecular minority spin stateshybridize with the Fe(001) minority spin states.
Figure 5(f) shows the minority spin states. The left-handside shows Fe(001) states. As shown in Fig. 3(a), Fe(001)
has a strong minority dxz+yz peak above the Fermi energy[the red line in Fig. 5(f)]. In the right-hand side in Fig. 5(f),HOMO/LUMO states of the freestanding H2 Pc are shownas black lines. When the H2 Pc approaches the Fe(001), theFe(001) minority spin state and H2 Pc LUMO minority spinstate are energetically close, and the chemisorption of the H2 Pcarises from the strong hybridization between the LUMO state
and the localized state at the Fe surface, which pushes up
the bonding and antibonding states in energy where electrons
between the H2 Pc and the Fe surface accumulate and deplete,respectively, as illustrated in I and IV in Fig. 5(h). On the other
hand, as shown in Fig. 5(g), the Fe(001) does not have the majority spin peak around the Fermi energy. Therefore, the molecular LUMO majority spin state is less affected. As a result, the
LUMO majority spin state remains above the Fermi energy.
By following this simple hybridization mechanism, it isspeculated that the experimentally observed peaks I and IV
are the hybridized minority spin states, and the peaks II and
III are the remaining LUMO majority spin state. Indeed, since
the peaks II and III, taken on molecules (at 0.14–0.18 eV),are located rather close to the surface states of the Fe(001)substrate (at around 0.17 eV), the spectroscopic features ofthe substrate may be retained to some extent on the molecule.
However, the cross-sectional views of the density distributionof the calculated interface LDOS, as mentioned in Fig. 5(h),may rule out the substrate contribution, since at I and IV, themolecular π and the Fe(001) d states are hybridized, while at II
and III, there is no hybridization. Thus, the substrate peaks haveno measurable contribution to the molecular peaks, and sucha unique hybridization is further supported at the Co(0001)
surface experimentally when they contact with π -conjugatedphthalocyanine molecules [51].
As already shown in Fig. 2, metal-free H2 Pc molecules
were observed as “single” molecules on Fe(001) at 300 K.
Spectroscopy measurements were able to be performed stably
at 300 K, while such metal-free molecules are known to be
diffused on noble metal substrates at 300 K and be formed
films [20,24]. Here, we remark that electronic interactions
determine the bonding strength between π -conjugated single
organic molecules and substrate electrodes, i.e., the insulating
electrode exhibits negligible electronic interaction with thesubstrate. Noble metal surfaces, in which mainly s or p states
are dominant near the Fermi energy (EF ), interact weakly withthe molecule, essentially through the process of physisorption.
Figure 6(a) shows that the LDOS of H2 Pc molecules
adsorbed on Ag(001), which is similar to the freestanding H2 Pc
in Figs. 5(a) and 5(b), i.e., the HOMO and LUMO states ofthe H2 Pc molecule, even after the molecule is adsorbed onto
the Ag(001) surface, do not change significantly from those of
the freestanding molecule. This leads to weak physisorption,and therefore single H2 Pc molecules easily overcome thediffusion barrier by thermal assistance, even at temperaturesless than room temperature.
The interfaces between organic molecules and 3d magnets
appear to be spin-polarized, even without the presence ofmagnetic ions in molecules due to hybridization between the dand π states, and thus, the molecules are likely chemisorbed.
Consequently, the distance between the substrate and themolecule is less than that in the physisorption case due to thestrong nature of chemisorbed bonds. Such chemisorption may
prevent the diffusion of single molecules and generate strong
bonding between single molecules and the electrode at 300 K.
Adsorption energy when the H2 Pc molecule approaches theAg(001) and Fe(001) was calculated. Here, Ag(001) was usedas the reference for the diffusion case.
The total energy as a function of the molecule-substrate
separation (d) for adsorption at the four-fold hollow (H) site
was calculated [Fig. 6(b)] under the absence of relaxation for
purposes of simplicity. The calculations clearly demonstratethat the adsorption energy on the Fe(001) surface (–4.2 eV at∼0.15 nm) is larger than that of the Ag(001) surface (–1.5 eV
at ∼0.2 nm) by a factor of about 3. (The adsorption energy
difference was assumed to be zero far from the surface at
430 pm.) It is noteworthy that the distances at the energy
minimum are approximately equal to the apparent height ofthe molecules in the STM images, which values are about
0.15 nm and 0.20 nm on the Fe(001) and Ag(001) surfaces,
respectively. These theoretical calculations indicate that asingle H2 Pc molecule does not diffuse on Fe(001) at 300 K,
while H2 Pc on Ag(001) is known to diffuse at 300 K.
To further confirm the robust adsorption of the H2 Pc
molecule onto the Fe(001) surface, the total energies atdifferent adsorption sites of the H2 Pc molecule, EH ,ET , EB1 ,and EB2 , on the Ag(001) and Fe(001) surfaces were calculated,assuming that the center of the molecule locates on the hollow
site (H), top site (T), bridge site 1 (B1), and bridge site 2(B2). In both surfaces, adsorption at the H site is found to
be energetically the most favorable. Importantly, the energy
differences of the T, B1, and B2 sites with respect to that of
the H site, ESite = ESite –EH , indicate that the H2 Pc/Fe(001)system presents a larger energy barrier to thermal diffusion
when compared with that of the H2 Pc/Ag(001) system.
For the Ag(001) surface, the energy difference is about
0.1–0.2 eV/molecule along the [110], [100], and [010]
directions, but for Fe(001), the corresponding values are 0.6,
0.9, and 1.0 eV, which values could thus be sufficiently
large to prevent the diffusion of the H2 Pc molecules even atroom temperature. By using the Arrhenius equation with thecalculated energy barriers, the average time that the molecules
spend before diffusing to neighboring sites, e.g., the T sites,
is confirmed to be significantly larger on the Fe substrate thanthat on the Ag substrate by a factor of 108 even in a room
temperature, where the frequency factor is assumed to be same
between the Fe and Ag substrates.
The thermal stability of H2 Pc molecules on Fe(001) was
experimentally confirmed by means of consecutively obtained
STM images of the same area. Figure 6(d) shows theseimages, which were sequentially acquired every 5 min. It is
obvious that single molecules do not diffuse on Fe(001) at
300 K, which validates the theoretical prediction indicated in
Figs. 6(b) and 6(c). The diffusion barrier on Fe(001) needs
to be four times that on Ag(001) to prevent diffusion. We
attempted to manipulate the single molecules by pushing themalong the [110] direction using the STM tip, because thisdirection has a lower barrier when compared with those of

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PHYSICAL REVIEW B 94, 195437 (2016)

the [100] and [010] directions [Fig. 6(c)]. The results of our“pushing” are shown in Fig. 6(e). In these cases, we attempted
to move five single molecules at different tunneling resistancesof 1 × 109  (Vs = –600 mV, I = 600 pA),2 × 108 (Vs =–600 mV, I =3 nA),2 × 107 (Vs =–600mV, I =30 nA), and3 × 106 (Vs = –100 mV, I = 30 nA); the molecules did not
move in any of the five cases.
Such a robust bonding that cannot be broken by STM tip
manipulation indicates the possibility of fabricating roomtemperature single-molecule junctions.

suggesting careful manners to deduce spin polarization of
the single molecules using STS. Studies of the H2 Pc single
molecules on Fe(001) first theoretically and experimentally
verify that systematically we can diffuse or immobilize theπ -conjugated single molecules at 300 K by selecting thecontacting substrate, which is key knowledge for realizationof 300-K single molecular devices. The H2 Pc/Fe(001) system
could be the first evidence to prove that the strong π -d bonding
can stop molecular diffusion at room temperature.
ACKNOWLEDGMENTS

Adsorption and electronic states of metal-free π -conjugatedphthalocyanine single molecules adsorbed on the Fe(001)whisker single crystal were studied by means of UHV-STM
and STS at 7 K and 300 K. The single H2 Pc molecules adsorbedwith three orientations. Type A is the most energetically stable,
parallel to [100], and type B is the second most stable, with arotation of about 23 deg from [100]. Rarely, type-C molecules
are observed with a rotation of about 45 deg from [100]. TypeC includes an extra atom at the core, possibly an Fe atom oran impurity atom. Types A and B have similar LDOS peaks.
Our ab initio calculations nicely recover the experimentallyobtained LDOS peaks, suggesting the hybridization process
between the molecular LUMO and the Fe(001) dxz+yz states.
Since Fe(001) has minority spin states above the Fermi energy,
hybridization mainly occurs with the LUMO minority spinstates, producing new bonding/antibonding states below/above
the Fermi energy. On the other hand, LUMO majority spinstates remain without hybridization by the d states; thus
spin-polarized LDOS peaks are tailored around the Fermi
energy. Further, even inside the same single molecule, core andarms (side groups) have different LDOS and spin polarization,

We thank Dr. D. T. Pierce (NIST) for his guidance regarding
the chemical vapor deposition of the Fe whisker. We thankProfessor Dr. H. Ishii (Chiba University) and Professor Dr. Y.
Noguchi (Meiji University) for supporting purification of themolecules, Professor Dr. F. Shibahara (Gifu University) for
NMR and IR measurements, and R. Nemoto for the Gaussian
fittings. Computations were performed at the Supercomputer
Center, Institute for Solid State Physics, University of Tokyo,
Center for Computational Materials Science, Institute forMaterials Research, Tohoku University, and Research Institutefor Information Technology, Kyushu University. This work
was supported by JSPS KAKENHI Grants No. 23681018 and
No. 25110011, the Japan Science and Technology Agency(JST) – Improvement of Research Environment for Young
Researchers, Chiba University Young Research – Oriented
Faculty Member Development Program in Bioscience Areas,
Casio Science Promotion Foundation, Asahi Glass Foundation, Hatakeyama Culture Foundation, The Association for
the Progress of New Chemistry, Iketani Science and Technology Foundation, The Noguchi Institute, Ozawa Yoshikawa
Memorial Electronics Foundation, Research Foundation for
the Electrotechnology of Chubu, The Nakajima Foundation,
The Futaba Electronics Memorial Foundation, and Shimadzu
Foundation.

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