The magnetic properties of the iron phthalocyanine molecule grafted to the Ti 2C MXene layer Aleksei KoshevarnikovTomi Ketolainen and Jacek A. Majewskiy

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The magnetic properties of the iron phthalocyanine molecule grafted to the Ti2C

MXene layer

Aleksei Koshevarnikov,∗Tomi Ketolainen, and Jacek A. Majewski†

Institute of Theoretical Physics, Faculty of Physics,

University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland

The magnetic tetrapyrrole molecules (such as porphyrins and phthalocyanines) with an active

transition metal atom in their centre are currently intensively studied as prosperous potential ele-

ments of devices for high-density information storage and processing. It has been recently proved

that by means of external factors one could induce two stable fully controllable molecular states.

Therefore, hybrid systems consisting of such magnetic molecules and suitable carriers from the

family of two-dimensional materials are often considered as promising highly scalable spintronic

systems that could in the near future lead to novel industrial applications. Here, we perform the

spin polarised density functional theory (DFT) studies of the hybrid system, which is the iron ph-

thalocyanine molecule (FePc) on the top of the titanium carbide Ti2C MXene layer. The most

relevant issue in this part is the interaction between magnetic atoms: Ti from MXene substrate and

iron from FePc. Four various magnetic conﬁgurations of FePc/Ti2C were considered. The signiﬁ-

cant ferromagnetic interaction between the iron atom and the upper titanium layer plays important

role in the reorientation of the iron atom’s magnetic moment. We also analyse a model of the sys-

tem in which the FePc molecule is in a quintet state (the ground state of an isolated molecule is a

triplet). To get a better understanding of the physics of the FePc/Ti2C hybrid system, we studied

the hybrid systems with a single iron atom and non-magnetic H2Pc on the Ti2C layer, Fe/Ti2C and

H2Pc/Ti2C, respectively, which nicely explains the role of the Pc ligand in the FePc/Ti2C hybrid

system.

I. INTRODUCTION

Transition metal phthalocyanines (TMPc’s) (Fig. 1)

are widely known organometallic compounds that have

been well known since the 1930s.[1] They consist of isoin-

dole rings interconnected via an sp2-hybridized nitrogen

atom and a transition metal atom in the centre. The ﬁrst

commercial implementation of these compounds was as

a blue pigment. Over the years, this type of molecule

has attracted attention of researches researchers in the

ﬁeld of molecular electronics. The ease of preparation,

the simple molecular structure, and the possibility of

functionalisation have made TMPc’s one of the leading

species for applications in the modern ﬁelds of spintron-

FIG. 1: The FePc molecule

∗aleksei.koshevarnikov@fuw.edu.pl

†jacek.majewski@fuw.edu.pl

ics and optoelectronics.[2–4] Such molecules can also act

as a part of an organic photodetector,[5] and as a hole

injection layer in OLED displays.[6]

MXene is a class of 2D compounds obtained from MAX

phases (layered hexagonal carbides or nitrides with a for-

mula Mn+1AXn, where n=1-3, M indicates a transitional

metal, A - elements mostly from groups 13 and 14, and X

- C or N atoms) by etching A elements. Firstly discovered

experimentally in 2011,[7] this class of compounds has

been developed rapidly. A lot of new species and their

functionalised derivatives have been synthesised. Soon

potential applications in thermoelectricity, catalysis, and

energy storage have been found.[8, 9] Here, we are in-

terested in the magnetic properties of MXenes and es-

pecially of the Ti2C (Fig. 2). It was shown [10] that

this layer exhibits several magnetic conﬁgurations where

the most stable are the conﬁgurations with co-directional

(ferromagnetic, Fig. 2b) and counter-directional (antifer-

romagnetic, 2c) magnetic moments of the Ti sublayers.

The energy of the antiferromagnetic conﬁguration is pre-

dicted to be lower just by 10 meV per primitive cell.

Because MXenes are relatively new materials, there

are not many quantum-chemical studies of the adsorp-

tion of molecules on the MXene surface. Experimental

studies of such systems are hampered by the fact that

the outer layers of the material adsorb substances from

the atmosphere, changing their chemical structure. Ex-

perimental studies of TMPc/MXene hybrid systems were

carried out taking into account that the surface is satu-

rated by atmospheric functional groups. In these cases,

MXenes lose their magnetic properties. Nonetheless, the

FePc/Ti3C2X2system was found to be a good catalyst

for redox reactions.[11] In addition, the same complex

arXiv:2210.02141v1 [cond-mat.mtrl-sci] 5 Oct 2022

(a)

(b) (c)

FIG. 2: The Ti2C 2D-threelayer system: (a) view from

above; and side views of (b) ferromagnetic and (c)

antiferromagnetic conﬁgurations. Red arrows indicate

the internal magnetic moment of Ti atoms.

works well in determining miRNAs and diagnosis of can-

cer biomarkers.[12] Computational studies of MXenes

have gone further than experimental ones. Quantum-

chemical methods make it possible to investigate com-

pounds that are still diﬃcult to obtain experimentally.

Theoretical studies of organometallic molecules on

MXene surfaces were not found in the literature, but

there are several theoretical studies devoted to the ad-

sorption of atoms on the surface of Ti2C and Ti3C22D

MXene layers. It was found [13] that the adsorption en-

ergies of 3d, 4d, and 5d transition metal atoms on Ti3C2

are in the range of -7.98 to -1.05 eV. 3d-transition met-

als on M2C layers (M = Ti, V, Cr, Zr, Nb, Mo, Hf,

Ta, and W) were studied [14] as single-atom catalysts to

ﬁnd an alternative to Pt-based catalysts. Also, there are

studies about metal atoms on functionalised layers for

single-atom catalysts [15, 16] and Li-ion storage.[17] It

should be noted that these studies did not take into ac-

count the spin polarisation of surfaces. The results of the

studies are presented in Table I. The calculation param-

eters given in parentheses will be explained in the next

chapter.

Determination of the magnetic moment of each atom

in a system is possible using spin-polarized scanning tun-

nelling microscopy.[18–20] In these works, the Fe atom

was studied in the interaction with InSb(110) surface. In

this study, the iron atom falls into the surface due to the

large lattice constant. In the case of the iron atom on top

of the Cu(001) surface, it was shown [21] that the elec-

tronic and magnetic properties of adatoms are strongly

aﬀected by the tip-surface distance.

In the purely theoretical study of iron chains on

Cu(001) and Cu(111) surfaces [22] a single iron atom acts

as a donor of spin momentum. It retains most of the spin

momentum when it is in the so-called ”ontop” position

and loses it when the atom is in or inside the surface.

The Bader charge transfer analysis for 3d TM atoms on

graphene and graphene/Ni(111) surfaces [23] indicates

that for all elements in the series electrons are trans-

ferred from the adatom to the graphene layer, leaving

the net charge on the adatom positive. Magnetic atoms

on the surface were also studied. Cobalt and iron atoms

were placed onto the Pt(111) surface [24] and also on

Pt(111) and Ir(111) surfaces.[25] There was shown that

adatoms induce polarisation on nearby surface atoms.

The magnetic anisotropy parameters also were calculated

and they are in agreement with the experimentally deter-

mined ones, where inelastic tunnelling spectroscopy was

implemented.[26]

The ability of an atom on a surface to have two sta-

ble states was studied in several articles. TM atoms on

the graphene/Ni(111) surface can be ferromagnetic and

antiferromagnetic toward the surface magnetisation.[23]

It was found that for Ti, V and Cr antiferromagnetic

alignment is preferable while for Mn, Fe and Co it is fer-

romagnetic (exchange energy for Fe, in this case, is 10

meV). By utilizing a combination of scanning tunnelling

spectroscopy and DFT methods, it was shown that a Co

atom on semiconducting black phosphorus [27] has two

states: low-spin and high-spin. It was shown experimen-

tally that the state of the atom can be switched elec-

trically. A holmium atom on the MgO surface exhibits

bistability property with up and down spin states.[28] It

was shown that it is possible to read the states using a

tunnel magnetoresistance and induce particular magnetic

state of the iron atom (just to ”write” it) with current

pulses using a scanning tunnelling microscope.

In this paper, we present the study of the FePc

molecule on the Ti2C MXene layer. Here, we mostly

focus on the magnetic properties of the formed system.

Diﬀerent magnetic conﬁgurations of the FePc/Ti2C hy-

brid system are modelled and compared. Additionally, a

single Fe atom and the H2Pc molecule on the top of Ti2C

are considered. It allows one to understand the role of

the phthalocyanine ligand in the complex.

II. COMPUTATIONAL DETAILS

The DFT calculations for the studied hybrid

FePc/Ti2C system with periodic boundary conditions

were performed employing the Quantum Espresso

6.5 numerical package [29] with the generalised gra-

dient approximation (GGA) [30] realised through

the Perdew-Burke-Ernzerhof (PBE) exchange-correlation

functional.[31] The van der Waals interaction between

the molecule and the Ti2C layer was accounted for within

the Grimme DFT-D3 ad-hoc scheme.[32] Rappe-Rabe-

Kaxiras-Joannopoulos (RRKJ) ultrasoft pseudopoten-

tials from pslibrary [33] were implemented. To treat the

strong on-site Coulomb interaction of TM d-electrons,

we used the DFT+U approach within the Hubbard

model.[34] The U parameter value for the Fe atom (U

TABLE I: Energetic, geometric and magnetic characteristics of Fe/Ti2C and Fe/Ti3C2hybrid systems. The sources

of the data are indicated. Adsorption sites for Ti3C2: Hcp - on the higher C-atom, Fcc - on the middle Ti; for Ti2C:

Hcp and Fcc - on the bottom Ti, but with diﬀerent symmetry surrounding.

Adsorption Energy, eV Fe height from the layer, ˚

Aµ(Fe)

Fe on Ti3C2(PBE)[13] Hcp: -3.849

Fcc: -3.847

Hcp: 2.373

Fcc: 2.360

—

Fe on Ti2C (PBE+D3)[14] Hcp: -4.093

Fcc: -3.897

Hcp: 1.708

Fcc: 1.644

Hcp: -2.03

Fcc: -2.28

= 4 eV) was taken from the previous results of linear

response calculations for TMPc molecules.[35] To simu-

late the excited state with S = 2, the Hubbard param-

eter U was changed from 4 to 6 eV. The kinetic energy

cutoﬀ for wavefunctions was set to 45 Ry and the cor-

responding parameter for charge density and potential

to 650 Ry. Preliminary estimations, optimisation, and

calculation of energy parameters were performed at the

Γ point. The chosen supercell for the FePc/Ti2C hybrid

system consisted of 7x7 Ti2C primitive cells. Such size of

the supercell makes it possible to place FePc molecules

at a distance of 6.36 ˚

A from each other, which practi-

cally excludes their spurious interaction. For the test

case of a single Fe atom on the Ti2C surface (Fe/Ti2C),

preliminary estimations, optimization, and calculation of

energy parameters were performed at the Γ point for 7x7

and 4x4 Ti2C supercells. For the case with the primitive

Ti2C cell, a denser grid of 5x5 k-points was used.

The adsorption energy Eawas determined according

to the formula

Ea=EF e(P c)+T i2C−EF e(P c)−ET i2C,(1)

where F e(P c) indicates either a single iron atom (F e) or

iron phthalocyanine (F eP c); T i2C- the substrate, and

F e(P c) + T i2Cthe whole system. To guarantee suitable

accuracy of Ea, the energies of these three systems are

calculated in the same supercell.

To investigate electron transfer, we performed an anal-

ysis of the laterally averaged electronic charge density

ρcharge =ρ↑+ρ↓and spin density ρspin =ρ↑−ρ↓, where

ρ↑and ρ↓are spin up and spin down electron charge den-

sities, respectively. The methodology of charge transfer

evaluation was inspired by the Bader charge analysis[36],

where the borders between the charge densities of two

atoms are determined along the local minimum of the

charge densities. Here, we consider the charge transfer

between the Ti2C surface and the ﬂat FePc molecule or

the iron atom. For this purpose, the charge density ρis

integrated over xy-planes in each point along the z-axis,

which gives laterally averaged charge density ¯ρ(z)

¯ρ(z) = 1

SZZS

ρ(x, y, z)dx dy, (2)

where S is the area of the unit cell. A similar approach

has been previously implemented to study charge distri-

bution in the hybrid system of VPc on gold surface.[37]

The minimum of the laterally averaged charge density

¯ρ(z) between the surface and the molecule could be then

considered as a surface (or line) dividing the regions of

the substrate and the molecule. The accurate minimum

was found using the polynomial approximation in the

vicinity of the boundary.

It is worth noting that the charge density analy-

sis was performed only for valence electrons qualiﬁed

as such in the employed pseudopotential. The cho-

sen pseudopotentials have the following valence con-

ﬁgurations: Ti(3s24s23p63d2), C(2s22p2), Fe(4s13d7),

N(2s22p3), H(1s1).

III. RESULTS AND DISSCUSSION

A. FePc

An isolated iron phthalocyanine molecule (Fig. 1) has

a tetragonal D4h symmetry. The central iron atom is in

the square planar ligand ﬁeld which is created by nitro-

gen atoms. The strong ligand ﬁeld makes the iron dx2−y2

orbital unfavourable, and, therefore, the ground state of

the molecule is triplet. The exact ground state is a topic

to study due to the energetic proximity of the two states.

While it seems to be commonly accepted that the ground

state of FePc is Egwith the iron 3d-shell conﬁguration

xyd2

xzd1

yzd1

z2d0

x2−y2, there exist computations predicting

A2gstate (with the d2

xyd1

xzd1

yzd2

z2d0

x2−y2iron 3d-shell con-

ﬁguration) as the ground.[38]

The excited quintet and singlet states can be modelled

using one-Slater-determinant DFT methods. Plane-wave

methods with the FePc molecule in the cubic cell with

the 20 ˚

A face show that the Fe-N bond lengths are 1.95

A in the ground state, 2.01 ˚

A in the quintet and 1.92

A in the singlet. This elongation of the bonds was pre-

viously found for molecules with the Fe-N4centre.[39]

The Fe-N bonds lengthening weakens the ligand ﬁeld cre-

ated by nitrogen atoms of tetragonal symmetry. Thus,

the electrons of the d-shell of the iron atom occupy the

higher energy orbitals. The quintet excited state is 0.44

eV higher than the triplet state and the singlet state is

2.68 eV higher.

The FePc symmetry is represented well in the cubic

cell. Below, FePc will be studied on the hexagonal lattice

structure surfaces. Therefore, the point symmetry of the

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ThemagneticpropertiesoftheironphthalocyaninemoleculegraftedtotheTi2CMXenelayerAlekseiKoshevarnikov,TomiKetolainen,andJacekA.MajewskiyInstituteofTheoreticalPhysics,FacultyofPhysics,UniversityofWarsaw,Pasteura5,02-093Warsaw,PolandThemagnetictetrapyrrolemolecules(suchasporphyrinsandphthalocyanines)wit...

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The magnetic properties of the iron phthalocyanine molecule grafted to the Ti 2C MXene layer Aleksei KoshevarnikovTomi Ketolainen and Jacek A. Majewskiy

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