Tuning and sensing spin interactions in Co-porphyrin/Au with $\mathrm{N}{\mathrm{H}}_{3}$ and $\mathrm{N}{\mathrm{O}}_{2}$ binding

Tuning and sensing spin interactions in Co-porphyrin/Au with $N H_{3}$ and $N O_{2}$ binding

Min Hui Chang, Yun Hee Chang, Na-Young Kim, Howon Kim, Soon-Hyeong Lee, Mahn-Soo Choi, Yong-Hyun Kim, and Se-Jong Kahng

Phys. Rev. B 100, 245406 – Published 6 December 2019

Abstract

Controlling spin interactions in magnetic-molecules/metal is essential for spintronic applications. Recent studies showed that using small molecule coordination, one could switch off the spin interactions between magnetic-molecules and metal substrates. However, this control should not be limited to the two-state switching. The strength of spin interaction can be reduced, but not “off” by the proper selection of small molecules. To demonstrate this, we considered two contrasting systems, $N H_{3}$ and $N O_{2}$ coordinated to Co-porphyrin/Au(111). In our scanning tunneling microscopy and spectroscopy (STM and STS), Kondo resonance was preserved with weakened spin coupling after $N H_{3}$ coordination. However, it disappeared after $N O_{2}$ coordination, implying “off” spin coupling. These observations are explained with our density functional theory calculation results. This study shows that small molecule coordination to magnetic-molecules/metal is a powerful way to control spin interactions at the single-molecule level.

Received 2 August 2019

DOI:https://doi.org/10.1103/PhysRevB.100.245406

Physics Subject Headings (PhySH)

Exchange interaction Kondo effect

Density functional theory Scanning tunneling microscopy Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Min Hui Chang^1,*, Yun Hee Chang^2,*, Na-Young Kim², Howon Kim ³, Soon-Hyeong Lee¹, Mahn-Soo Choi¹, Yong-Hyun Kim^2,4,†, and Se-Jong Kahng^1,‡

¹Department of Physics, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea
²Graduate School of Nanoscience and Technology, KAIST, Daejeon 34141, Republic of Korea
³Department of Physics, University of Hamburg, D-20355 Hamburg, Germany
⁴Department of Physics, KAIST, Daejeon 34141, Republic of Korea

^*These authors contributed equally.
^†yong.hyun.kim@kaist.ac.kr
^‡sjkahng@korea.ac.kr

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Vol. 100, Iss. 24 — 15 December 2019

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Figure 1
Controlling spin exchange interactions. Schematic diagrams to depict how spin exchange interactions between metalloporphyrin and Au(111) can be controlled by small molecule coordination. The charge donation ability of small molecules is known to be vacuum $< N H_{3} < N O_{2}$ . With $N H_{3}$ coordination, the exchange interaction will be weakened because of the increased distance between metalloporphyrin and Au(111), resulting in reduced Kondo temperature. With $N O_{2}$ coordination, the exchange interaction will be even more weakened, resulting in absent Kondo resonance.
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Figure 2
$N H_{3}$ and $N O_{2}$ coordination to CoTPP on Au(111). (a), (b) Typical STM images of mixed molecular islands of CoTPP and $H_{2} TPP$ on the Au(111) surface after exposure to $N H_{3}$ gas. (c), (d) STM images of mixed molecular islands of CoTPP and $H_{2} TPP$ on the Au(111) surface after exposure to $N O_{2}$ gas. The four representative molecules of CoTPP, $H_{2} TPP, N H_{3}$ -CoTPP, and $N O_{2}$ -CoTPP are denoted by blue, green, red, and pink marks, respectively, in (b) and (d). Inset of (a): Ball and stick models of CoTPP and $H_{2} TPP$ molecules. The cyan, blue, red, and white circles denote carbon, nitrogen, cobalt, and hydrogen atoms, respectively. The sizes of STM images: (a) and (c) $96 \times 96 n m^{2}$ , (b) and (d) $11 \times 11 n m^{2}$ . Tunneling current: $I_{T} = 0.1 nA$ . Sample voltage: (a), (c), (d) $V_{S} = - 0.3 V$ , (b) $V_{S} = - 0.2 V$ .
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Figure 3
Desorption of $N H_{3}$ and $N O_{2}$ from CoTPP. (a), (d) A current-voltage (I-V) curve obtained during STM manipulation procedure for (a) $N H_{3}$ and (d) $N O_{2}$ detachment. At the center of a gas-adsorbed porphyrin molecule, the sample bias was swept while holding the feedback loop open, resulting in an abrupt decrease in current at (a) + 0.85 V and (d) + 0.92 V. The STM images obtained (b) and (e) before and (c) and (f) after the STM manipulation of (a) and (d), respectively. The size of STM images: $4.5 \times 4.5 n m^{2}$ ; tunneling current: $I_{T} = 0.1 nA$ ; sample voltage: $V_{S} = - 0.3 V$ .
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Figure 4
Kondo resonance peaks. (a)–(c) Scanning tunneling spectroscopy spectra for (a) CoTPP, (b) $N H_{3}$ -CoTPP, and (c) $N O_{2}$ -CoTPP obtained at the center of three individual CoTPP, $N H_{3}$ -CoTPP, and $N O_{2}$ -CoTPP denoted in the insets. The initial tunneling conditions are $I_{T} = 0.1 nA$ and Sample voltage: $V_{S} = - 0.25 V$ . The blue and red solid lines show a Fano fitting. The fitting parameters for the blue solid lines are $q = 14, ε_{K} = - 24 meV$ , and $T_{K} = 205 K$ . The fitting parameters for the red solid lines are $q = 55, ε_{K} = - 15 meV$ , and $T_{K} = 135 K$ . (Fitting formulas are in supporting information).
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Figure 5
Calculated geometric and electronic structures. (a), (c) DFT stabilized atomic model of vertical coordinated (a) $N H_{3}$ and (c) $N O_{2}$ molecules on Au-supported CoTPP molecule. (b), (d) Partial density of states (PDOS) for the coordinated molecule. The Fermi level is set to be zero. Red and blue lines indicate majority and minority spin of Co $d$ -orbital, respectively. Green lines indicate spin (b) $N H_{3}$ and (d) $N O_{2} p$ -orbital.
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Figure 6
Calculated spin distributions. (a), (c) Calculated spatial spin density plots for (a) CoTPP and (c) $N H_{3}$ -CoTPP molecule. (b), (d) Integrated spin density plot along the $z$ axis for (b) CoTPP and (d) $N H_{3}$ -CoTPP molecule. The $z$ coordinate zero means the top Au atom position.
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Physical Review B

covering condensed matter and materials physics

Tuning and sensing spin interactions in Co-porphyrin/Au with $N H_{3}$ and $N O_{2}$ binding

Min Hui Chang, Yun Hee Chang, Na-Young Kim, Howon Kim, Soon-Hyeong Lee, Mahn-Soo Choi, Yong-Hyun Kim, and Se-Jong Kahng

Phys. Rev. B 100, 245406 – Published 6 December 2019

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Physical Review B

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Tuning and sensing spin interactions in Co-porphyrin/Au with NH3 and NO2 binding

Min Hui Chang, Yun Hee Chang, Na-Young Kim, Howon Kim, Soon-Hyeong Lee, Mahn-Soo Choi, Yong-Hyun Kim, and Se-Jong Kahng

Phys. Rev. B 100, 245406 – Published 6 December 2019

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Tuning and sensing spin interactions in Co-porphyrin/Au with $N H_{3}$ and $N O_{2}$ binding