Correlative Scanning Probe Microscopy
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Studying novel materials and phases is arguably the most rewarding research field, as it often yields novel technological applications. While serendipity has played a role on such paths, a prerequisite for the design of novel materials with desired functionalities is a systematic study of their properties.

Usually various properties have to be examined under a variety of conditions in order for the material behaviour to be understood. Correlative scanning probe microscopy (SPM) is proven to be an essential tool therein, because it enables correlating material properties - like magnetization, electric polarization, Kelvin potential, electrical conductivity, and topography - at different locations of the sample surface, and also at various temperatures & magnetic fields. This is why attocube provides its customers with various upgrades for its cryogenic attoAFM I microscope, as reliable features for correlative measurements:

MFM is the most used upgrade, and we featured it many times up to now. This is why we want to focus on other upgrades in this newsletter. Below we present some of recent remarkable measurements obtained with attoAFM I & its upgrades by our customers, who measured and correlated electric polarization (PFM), Kelvin potential quantitatively (KPFM), Kelvin potential qualitatively (EFM), electrical conductivity (ct-AFM), and topography (topo). 

KPFM, EFM, PFM & TOPO
Elucidated Behavior of a Ferroelectric-Semiconductor Phototransistor

Low-light-level photodetectors (3LPDs) are key ingredients for quantum photonics devices, as well as in astronomy. To be compatible with complementary metal oxide semiconductors (CMOS) technology, they need to operate at low voltages, which is not the case as of now.

The team lead by Zhihai Cheng (Renmin University of China, China) and Zhenxing Wang (National Center for Nanoscience and Technology, China) has fabricated and characterized low-light-level ferroelectric-semiconductor phototransistor (FSP) with an intrinsically high gain, featuring photo-induced ferroelectric switching.

To unravel the photoresponse mechanism, they carried out in-situ EFM and KPFM on the FSP devices, in which the ferroelectric-semiconductor channels were identified by PFM.

These correlative measurements have been realized by an attoAFM I microscope in an attoDRY2100 cryostat, with a KPFM and a PFM upgrade. This FSP demonstrates potential for novel generation of 3LPDs, due to its low operating voltage, high performance and simple architecture.

ct-AFM, PFM & TOPO
Conducting Domain Walls in Quantum Materials

Conducting domain walls (DWs) are quasi-2D conducting pathways that can be created, positioned, and removed in-situ, giving an opportunity for rewritable nanoelectronics. In this young field, conducting DWs typically arise in wide band-gap ferroelectrics where they can form in response to charge build up at polar discontinuities. The chair of István Kézsmárki (University of Augsburg, Germany) has shown that conducting DWs can also exist in narrow-gap Mott insulators. In this case, it was shown that the nanoscale conducting pathways form because of strain gradients around the DWs that change the band structure.

The team used an attoAFM I microscope with a ct-AFM upgrade and a PFM upgrade in an attoLIQUID2000 cryostat to cool their template material (GaV4S8) below its Jahn-Teller transition (~43 K) before directly imaging the conductivity, the topography, and the piezoresponse. From this they were able to rule out the polar discontinuity model as the origin, rather correlating the increase in conductivity around the DWs to the square of height of the surface reconstruction: a signature of the volume strain that arises across the Jahn-Teller transitions. Effectively, this shows a new mechanism for creating nanoscale conducting pathways using strain-gradient-induced changes in band structure. This opens the exciting door for many new materials to be considered for domain wall nanoelectronics.

PFM & TOPO
Emergent Magnetoelectric Phase Transition

Complex oxides with a broken symmetry can exhibit various, often emergent, phases. This is in particular facilitated by designing superlattices of complex oxides. The group of Jinxing Zhang (Beijing Normal University, China) has constructed a superlattice by alternately stacking a Ruddlesden–Popper and a perovskite oxide, which resulted in artificially designed ferroelectricity and emergent magnetoelectric (ME) phase transition.

The existence of ferroelectric domains below 90 K has been verified by PFM, by using an attoAFM I microscope with a PFM upgrade in an attoDRY1000 cryostat. The accompanying existence of Dzyaloshinskii–Moriya interaction (DMI) and net magnetization have been verified by Brillouin light scattering. Furthermore, external magnetic field suppresses electrical polarization, which confirms the existence of a direct ME effect. This study shows that engineering of interfacial DMI is a promising tool for generating exotic phases and orders in systems with correlated electrons.