Final Report Summary - ELECTROMAGRAPHENE (Probing the influence of the graphene-metal interaction on the electronic and magnetic properties of nearly free standing graphene)
The proposed research was aimed at the consecution of three objectives. In the following we will describe the work carried out to achieve these project's objectives as well as the main results obtained.
Objective 1: To achieve the ultimate in decoupling and isolating graphene monolayers of perfect structure from their local environment:
Using Scanning Tunneling Microscopy (STM) in Ultrahigh Vacuum (UHV), the oxygen adsorbate superstructures on Ir(111) have been identified and compared to the ones formed by intercalation in between graphene (Gr) and the Ir(111) substrate. Our atomically resolved STM images reveal the existence of two structures which are only observed when the Gr layer is on top. Importantly, these new phases do not have rotational domains. Therefore, intercalated layers exhibiting these arrangements present the highest symmetry reported till now for the Gr/O/Ir(111) interface. Here we should mention that we have written a manuscript which has been submitted to Nanoscale.
Objective 2: Characterization of the structural and electronic properties of such nearly free-standing graphene monolayers:
We have performed a microscopic in situ analysis in UHV by STM measurements carried out at 20 K of our Gr/O/Ir(111) samples reported in the objective 1. In particular, we have observed well extended patterns arising from intervalley scattering processes of charge carriers in Gr generated around defects. These modulations have been previously observed in the surroundings of defects in other Gr like systems well decoupled from their respective substrates. Then, the observation of these scattering processes is an indication that indeed Gr monolayers are electronically well decoupled from the Ir(111) substrate due to the oxygen intercalated layer.
Objective 3: To design and characterize graphene-metal composites with new functionalities:
We have developed two kinds of Gr based nanomaterials:
- Gr/Rare earth monolayer/ferromagnetic thin film/Ir(111): The unique properties of Gr turn it into a very interesting material for applications in spintronics. Then, the development of techniques aimed to bestow Gr itself with magnetic properties it is an interesting issue. The choice of rare earth elements was made due to the fact that they have f-orbitals exhibiting high magnetic moments and they bind to Gr in such way that Gr band structure remains intact. This is in contrast to, e.g. the case of Gr on Ni, where the formation of covalent bonds between Ni and Gr destroys the key feature of Gr's band structure, the Dirac cone.
Then, we firstly started to study the growth in situ in UHV of intercalated rare earth monolayers of Eu and Nd in the Gr/Ir(111) interface. By means of Low Energy Electron Microscopy (LEEM) and Photoemission Electron Microscopy (PEEM) experiments in UHV performed in collaboration with Dr. Frank-J. Meyer zu Heringdorf and prof. Michael Horn-von Hoegen we have observed in real time that in full layer Gr grown on Ir(111) the Eu penetration takes place at wrinkles via nanoscale cracks. This result has been published in Physical Review B.
Afterwards, we grew also in UHV Gr/Eu/Co/Ir(111), Gr/Eu/Ni/Ir(111) and Gr/Nd/Co/Ir(111) systems. The samples with Eu-intercalated Gr on Ni and Co thin films were investigated by X-Ray Magnetic Circular Dichroism (XMCD) conducted at the PM3 bending magnet beamline of the BESSY II synchrotron facility in collaboration with the group of Prof. Heiko Wende. The XMCD measurements show that the Eu monolayer has a net moment, which is anti-parallel to the Ni/Co moment. Additionally, our results have demonstrated that the ordering of the moment of the Eu monolayer is induced by an antiferromagnetic coupling with the Ni film. Unfortunately, we could not measure any reliable XMCD signal coming from the Gr layer meaning that either there is not a net magnetic moment induced on Gr or it is below our detection limit. However, the presence of the Gr layer is still crucial for future applications since it protects the Eu layer against oxidation leaving intact the magnetic properties of the rare earth/Ni (or Co) thin film/Ir(111) system. With these results a manuscript is being written which is intended to be submitted to Physical Review Letters.
- Cluster arrays with high thermal stability: A previous work performed in our group demonstrated that on the Gr moiré on Ir(111) a variety of highly perfect cluster superlattices with a periodicity of 2.5 nm can be grown as shown for Ir, Pt, W and Re. Even materials that do not form cluster superlattices upon room temperature deposition may be grown into such by low-temperature deposition or the application of cluster seeding through Ir. It has allowed to grow clusters superlattices also of magnetic elements as Co or Fe. However, some time ago it was demonstrated that those clusters superlattices are stable only up to 450K. Here, it should be mentioned that for applications of cluster superlattices in fields as nanomagnetism and nanocatalysis thermal stability of the cluster arrays and the absence of sintering at the temperature of use are of crucial importance. We have developed two approaches for improving thermal stabilization of cluster superlattices. The first method consisted in the exposure of the cluster superlattice to atomic oxygen. In this case cluster superlattices remains stable up to 600 K. The second method was to embed the cluster superlattice into an aluminum oxide matrix. Although further measurements are still needed, we intend to write a manuscript that it will be submitted to Nano Letters.
Additionally, we have considered two new objectives which were not initially included in the
proposed research:
- Additional objective 1: Understanding the origin of the formation of bulges in Gr on Ir(111) after ion irradiation.
Recently, it was reported by some of the members of the group the formation of bulges in a Gr sheet epitaxially grown on Ir(111) after conducting irradiation with Xe ions (energy range of 0.1-5 keV) at 300 K and subsequent annealing to 1000 K. In that previous work it was ruled out that the bulges are agglomerations of implanted noble gas atoms. In particular, it was stated that although noble gas is certainly implanted into the Ir crystal, trapped in bulk vacancies as well as bulk vacancy clusters, and partially released during annealing to 1000 K, the Gr cover does not protect it from desorption. Additional X-Ray Photoemission Spectroscopy (XPS) experiments, performed in MAXII synchrotron in collaboration with Dr. Jan Knudsen, after irradiation and annealing show that indeed the Gr cover protects Xe efficiently from desorption. By using these results a comment to the previous paper has been published in ACS Nano.
- Additional objective 2: Tuning the van der Waals interaction of Gr with organic adsorbates.
We have shown, by a combination of STM and Thermal Desorption Spectroscopy (TDS) experiments performed in our group and theoretical calculations carried out by the group of Prof. Stefan Blügel, that the binding of benzene and naphthalene to Gr is weakened or strengthened via p- or n-doping upon intercalation of O and Eu at the Gr/Ir(111) interface. Based on the robustness of the effect with respect to changing either the intercalant or the molecule, and the correlation of the binding energy with the amount of charge transfer into Gr, we thus propose that the van der Waals interaction of Gr can be effectively tuned by doping. These results have allowed us to write a manuscript that will be submitted within the next weeks to Physical Review Letters.
Objective 1: To achieve the ultimate in decoupling and isolating graphene monolayers of perfect structure from their local environment:
Using Scanning Tunneling Microscopy (STM) in Ultrahigh Vacuum (UHV), the oxygen adsorbate superstructures on Ir(111) have been identified and compared to the ones formed by intercalation in between graphene (Gr) and the Ir(111) substrate. Our atomically resolved STM images reveal the existence of two structures which are only observed when the Gr layer is on top. Importantly, these new phases do not have rotational domains. Therefore, intercalated layers exhibiting these arrangements present the highest symmetry reported till now for the Gr/O/Ir(111) interface. Here we should mention that we have written a manuscript which has been submitted to Nanoscale.
Objective 2: Characterization of the structural and electronic properties of such nearly free-standing graphene monolayers:
We have performed a microscopic in situ analysis in UHV by STM measurements carried out at 20 K of our Gr/O/Ir(111) samples reported in the objective 1. In particular, we have observed well extended patterns arising from intervalley scattering processes of charge carriers in Gr generated around defects. These modulations have been previously observed in the surroundings of defects in other Gr like systems well decoupled from their respective substrates. Then, the observation of these scattering processes is an indication that indeed Gr monolayers are electronically well decoupled from the Ir(111) substrate due to the oxygen intercalated layer.
Objective 3: To design and characterize graphene-metal composites with new functionalities:
We have developed two kinds of Gr based nanomaterials:
- Gr/Rare earth monolayer/ferromagnetic thin film/Ir(111): The unique properties of Gr turn it into a very interesting material for applications in spintronics. Then, the development of techniques aimed to bestow Gr itself with magnetic properties it is an interesting issue. The choice of rare earth elements was made due to the fact that they have f-orbitals exhibiting high magnetic moments and they bind to Gr in such way that Gr band structure remains intact. This is in contrast to, e.g. the case of Gr on Ni, where the formation of covalent bonds between Ni and Gr destroys the key feature of Gr's band structure, the Dirac cone.
Then, we firstly started to study the growth in situ in UHV of intercalated rare earth monolayers of Eu and Nd in the Gr/Ir(111) interface. By means of Low Energy Electron Microscopy (LEEM) and Photoemission Electron Microscopy (PEEM) experiments in UHV performed in collaboration with Dr. Frank-J. Meyer zu Heringdorf and prof. Michael Horn-von Hoegen we have observed in real time that in full layer Gr grown on Ir(111) the Eu penetration takes place at wrinkles via nanoscale cracks. This result has been published in Physical Review B.
Afterwards, we grew also in UHV Gr/Eu/Co/Ir(111), Gr/Eu/Ni/Ir(111) and Gr/Nd/Co/Ir(111) systems. The samples with Eu-intercalated Gr on Ni and Co thin films were investigated by X-Ray Magnetic Circular Dichroism (XMCD) conducted at the PM3 bending magnet beamline of the BESSY II synchrotron facility in collaboration with the group of Prof. Heiko Wende. The XMCD measurements show that the Eu monolayer has a net moment, which is anti-parallel to the Ni/Co moment. Additionally, our results have demonstrated that the ordering of the moment of the Eu monolayer is induced by an antiferromagnetic coupling with the Ni film. Unfortunately, we could not measure any reliable XMCD signal coming from the Gr layer meaning that either there is not a net magnetic moment induced on Gr or it is below our detection limit. However, the presence of the Gr layer is still crucial for future applications since it protects the Eu layer against oxidation leaving intact the magnetic properties of the rare earth/Ni (or Co) thin film/Ir(111) system. With these results a manuscript is being written which is intended to be submitted to Physical Review Letters.
- Cluster arrays with high thermal stability: A previous work performed in our group demonstrated that on the Gr moiré on Ir(111) a variety of highly perfect cluster superlattices with a periodicity of 2.5 nm can be grown as shown for Ir, Pt, W and Re. Even materials that do not form cluster superlattices upon room temperature deposition may be grown into such by low-temperature deposition or the application of cluster seeding through Ir. It has allowed to grow clusters superlattices also of magnetic elements as Co or Fe. However, some time ago it was demonstrated that those clusters superlattices are stable only up to 450K. Here, it should be mentioned that for applications of cluster superlattices in fields as nanomagnetism and nanocatalysis thermal stability of the cluster arrays and the absence of sintering at the temperature of use are of crucial importance. We have developed two approaches for improving thermal stabilization of cluster superlattices. The first method consisted in the exposure of the cluster superlattice to atomic oxygen. In this case cluster superlattices remains stable up to 600 K. The second method was to embed the cluster superlattice into an aluminum oxide matrix. Although further measurements are still needed, we intend to write a manuscript that it will be submitted to Nano Letters.
Additionally, we have considered two new objectives which were not initially included in the
proposed research:
- Additional objective 1: Understanding the origin of the formation of bulges in Gr on Ir(111) after ion irradiation.
Recently, it was reported by some of the members of the group the formation of bulges in a Gr sheet epitaxially grown on Ir(111) after conducting irradiation with Xe ions (energy range of 0.1-5 keV) at 300 K and subsequent annealing to 1000 K. In that previous work it was ruled out that the bulges are agglomerations of implanted noble gas atoms. In particular, it was stated that although noble gas is certainly implanted into the Ir crystal, trapped in bulk vacancies as well as bulk vacancy clusters, and partially released during annealing to 1000 K, the Gr cover does not protect it from desorption. Additional X-Ray Photoemission Spectroscopy (XPS) experiments, performed in MAXII synchrotron in collaboration with Dr. Jan Knudsen, after irradiation and annealing show that indeed the Gr cover protects Xe efficiently from desorption. By using these results a comment to the previous paper has been published in ACS Nano.
- Additional objective 2: Tuning the van der Waals interaction of Gr with organic adsorbates.
We have shown, by a combination of STM and Thermal Desorption Spectroscopy (TDS) experiments performed in our group and theoretical calculations carried out by the group of Prof. Stefan Blügel, that the binding of benzene and naphthalene to Gr is weakened or strengthened via p- or n-doping upon intercalation of O and Eu at the Gr/Ir(111) interface. Based on the robustness of the effect with respect to changing either the intercalant or the molecule, and the correlation of the binding energy with the amount of charge transfer into Gr, we thus propose that the van der Waals interaction of Gr can be effectively tuned by doping. These results have allowed us to write a manuscript that will be submitted within the next weeks to Physical Review Letters.