their chemical compositions in the supercell are Cu 108 Ta 32 O 80 Pt 84 ,
Cu 108 Ta 32 O 88 Pt 84 and Cu 108 Ta 32 O 96 Pt 84 , and hereafter they are denoted as a-O8,
a-O12 and a-O16, respectively. These three models have different interface O
concentrations: a-O8 represents the initial Cu/c-Ta 2 O 5 /Pt structure with a stoichiometric c-Ta 32 O 80 slab between two electrodes, where O layers of both Cu/c-Ta 2 O 5
and Pt/c-Ta 2 O 5 interfaces contain 8 O atoms; In the a-O12 and a-O16 models, extra
4 and 8 O atoms are introduced into each interface, respectively. In preparing the
above simulation models, Cu/crystal (c)-Ta 2 O 5 /Pt heterostructures were first
constructed with corresponding amounts of interface O atoms, and melt-quenching
cycle was performed to generate a-Ta 2 O 5 layers with fixing the positions of atoms in
the electrodes. Then the obtained Cu/a-Ta 2 O 5 /Pt heterostructures were further equilibrated at room temperature (unless otherwise described), which was followed by the
structural optimization with relaxing all the atoms.
The as-generated three Cu/a-Ta 2 O 5 /Pt heterostructures are shown in Fig. 7a–c. In
the stoichiometric case (a-O8, Fig. 7a), we can see the accumulation of O atoms near
the Cu/a-Ta 2 O 5 interface with forming Cu-O bonds and their depletion near the Pt/aTa 2 O 5 interface where a considerable number of Pt-Ta bonds are found. Most of Cu
atoms at the Cu/a-Ta 2 O 5 interface are connected with single O atom. Their Cu-O
bond length is about 2.15 Å in average, which is longer than that in the bulk Cu 2 O
(1.85 Å). According to the Bader charge analyses (Fig. 8a), the ionization of Cu
atoms at this interface occurs with the charge transfer from Cu to O atom of about
0.2e in average. Some Cu atoms at the interface are significantly ionized with
forming two Cu-O bonds, accompanied by the charge transfer from Cu to O about
0.4e. Interestingly, these Cu atoms tend to diffuse into a-Ta 2 O 5 bulk layer. On the
other hand, as seen in Fig. 8a, the charge transfer occurs from Ta to Pt atoms at the
Pt/a-Ta 2 O 5 interface by forming Ta-Pt bonds. This Ta-Pt bond formation involves
the decrease in the number of Ta-O bonds from TaO 5 to TaO 4 , which results in the
reduction of Ta atoms.
As the O content at the interface region increases, the interface structures of Cu/aTa 2 O 5 show larger distortion as seen in Fig. 7b (a-O12) and 7c (a-O16). Interestingly, Ta-O-O bonding structures are seen at the interface region due to the high
interface O content, and the O ions in Ta-O-O bond hold less amount of electrons
than the others, as can be seen in the Bader charges in a-O16 model (Fig. 8b). It is
worth noting that a similar Ta-O-O bonding structure has been identified in Cu/aTa 2 O 5 /Pt atomic switch in experiment [25]. It is also noted that, in the a-O12 and
a-O16 structures, considerable amount of Cu atoms at the Cu/a-Ta 2 O 5 interface have
diffused into the a-Ta 2 O 5 bulk region with forming two (predominant) or three Cu-O
bonds. The corresponding Cu-O bond length is about 1.90 Å in average, which is a
little longer than that in bulk Cu 2 O (1.85 Å). For example, in the a-O16 structure, the
Cu atoms at the interface region are oxidized with losing about 0.55 electrons in
average (Fig. 8b). Since the Bader charge values in the Cu/a-Ta 2 O 5 /Pt systems are
about half of the corresponding formal values (for example, the Bader charges of Ta
and O atom are about +2.5 and À1e, respectively, while the formal charges are Ta
(+5e) and O (À2e) ions in a-Ta 2 O 5 ), the Cu atoms at the interface region with 0.55e
could be regarded as Cu
1.1+ .
104
S. Watanabe and B. Xiao
Cu 108 Ta 32 O 88 Pt 84 and Cu 108 Ta 32 O 96 Pt 84 , and hereafter they are denoted as a-O8,
a-O12 and a-O16, respectively. These three models have different interface O
concentrations: a-O8 represents the initial Cu/c-Ta 2 O 5 /Pt structure with a stoichiometric c-Ta 32 O 80 slab between two electrodes, where O layers of both Cu/c-Ta 2 O 5
and Pt/c-Ta 2 O 5 interfaces contain 8 O atoms; In the a-O12 and a-O16 models, extra
4 and 8 O atoms are introduced into each interface, respectively. In preparing the
above simulation models, Cu/crystal (c)-Ta 2 O 5 /Pt heterostructures were first
constructed with corresponding amounts of interface O atoms, and melt-quenching
cycle was performed to generate a-Ta 2 O 5 layers with fixing the positions of atoms in
the electrodes. Then the obtained Cu/a-Ta 2 O 5 /Pt heterostructures were further equilibrated at room temperature (unless otherwise described), which was followed by the
structural optimization with relaxing all the atoms.
The as-generated three Cu/a-Ta 2 O 5 /Pt heterostructures are shown in Fig. 7a–c. In
the stoichiometric case (a-O8, Fig. 7a), we can see the accumulation of O atoms near
the Cu/a-Ta 2 O 5 interface with forming Cu-O bonds and their depletion near the Pt/aTa 2 O 5 interface where a considerable number of Pt-Ta bonds are found. Most of Cu
atoms at the Cu/a-Ta 2 O 5 interface are connected with single O atom. Their Cu-O
bond length is about 2.15 Å in average, which is longer than that in the bulk Cu 2 O
(1.85 Å). According to the Bader charge analyses (Fig. 8a), the ionization of Cu
atoms at this interface occurs with the charge transfer from Cu to O atom of about
0.2e in average. Some Cu atoms at the interface are significantly ionized with
forming two Cu-O bonds, accompanied by the charge transfer from Cu to O about
0.4e. Interestingly, these Cu atoms tend to diffuse into a-Ta 2 O 5 bulk layer. On the
other hand, as seen in Fig. 8a, the charge transfer occurs from Ta to Pt atoms at the
Pt/a-Ta 2 O 5 interface by forming Ta-Pt bonds. This Ta-Pt bond formation involves
the decrease in the number of Ta-O bonds from TaO 5 to TaO 4 , which results in the
reduction of Ta atoms.
As the O content at the interface region increases, the interface structures of Cu/aTa 2 O 5 show larger distortion as seen in Fig. 7b (a-O12) and 7c (a-O16). Interestingly, Ta-O-O bonding structures are seen at the interface region due to the high
interface O content, and the O ions in Ta-O-O bond hold less amount of electrons
than the others, as can be seen in the Bader charges in a-O16 model (Fig. 8b). It is
worth noting that a similar Ta-O-O bonding structure has been identified in Cu/aTa 2 O 5 /Pt atomic switch in experiment [25]. It is also noted that, in the a-O12 and
a-O16 structures, considerable amount of Cu atoms at the Cu/a-Ta 2 O 5 interface have
diffused into the a-Ta 2 O 5 bulk region with forming two (predominant) or three Cu-O
bonds. The corresponding Cu-O bond length is about 1.90 Å in average, which is a
little longer than that in bulk Cu 2 O (1.85 Å). For example, in the a-O16 structure, the
Cu atoms at the interface region are oxidized with losing about 0.55 electrons in
average (Fig. 8b). Since the Bader charge values in the Cu/a-Ta 2 O 5 /Pt systems are
about half of the corresponding formal values (for example, the Bader charges of Ta
and O atom are about +2.5 and À1e, respectively, while the formal charges are Ta
(+5e) and O (À2e) ions in a-Ta 2 O 5 ), the Cu atoms at the interface region with 0.55e
could be regarded as Cu
1.1+ .
104
S. Watanabe and B. Xiao