Ali Dadgar - Academia.edu (original) (raw)

Papers by Ali Dadgar

APS March Meeting Abstracts, 2016

Bulletin of the American Physical Society, Mar 15, 2016

Bulletin of the American Physical Society, Mar 18, 2016

Bulletin of the American Physical Society, Mar 17, 2016

Bulletin of the American Physical Society, Mar 15, 2016

Nano Letters, Apr 27, 2016

Bulletin of the American Physical Society, Mar 5, 2015

Bulletin of the American Physical Society, Mar 18, 2016

arXiv (Cornell University), Mar 16, 2023

Nano Letters, May 23, 2017

High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such a... more High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such as MoS 2 is key to unlocking their unique electronic and optoelectronic properties for fundamental research and device applications. Despite extensive experimental and theoretical efforts reliable ohmic contact to doped TMDCs remains elusive and would benefit from a better understanding of the underlying physics of the metal-TMDC interface. Here we present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer MoS 2 using ultra-high vacuum scanning tunneling microscopy (UHV-STM). Our measurements reveal that the electronic properties of these junctions are dominated by 2D metal induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (L-DOS) of the MoS 2 within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of ~0.55 nm near mid gap to as long as 2 nm near the band edges and are nearly identical for Au, Pd and graphite contacts, indicating that it is a universal property of the monolayer semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact.

Bulletin of the American Physical Society, 2017

high-quality electrical contact between deposited metals and monolayer transition metal dichalcog... more high-quality electrical contact between deposited metals and monolayer transition metal dichalcogenides remains a challenging problem. In this work, we probe the factors influencing the contact resistance by using atomicresolution scanning probe microscopy and spectroscopy at the interface between a contact metal and highly doped monolayer molybdenum disulfide (MoS 2). Using atomically resolved spectroscopy measurements, band structure evolution is mapped across the edge of the contact and into the MoS 2 sheet. The spectroscopy shows the presence of metal induced gap states (MIGS) in the semiconductor with a decay length of a few angstroms from the interface, which is the first experimental observation of MIGS in a two-dimensional semiconductor. We show that in the highly doped limit, the MIGS dominate the electronic properties and hence the contact resistance, as opposed to band bending. Finally, we describe the difference in MIGS between graphite, gold and palladium electrodes on MoS 2 .

Bulletin of the American Physical Society, 2016

Bulletin of the American Physical Society, 2016

Graphene p-n junctions are essential devices for studying relativistic Klein tunneling and the Ve... more Graphene p-n junctions are essential devices for studying relativistic Klein tunneling and the Veselago lensing effect in graphene. We have successfully fabricated graphene p-n junctions using both lithographically pre-patterned substrates and the stacking of vertical heterostructures. We then use our 4-probe STM system to characterize the junctions. The ability to carry out scanning electron microscopy (SEM) in our STM instrument is essential for us to locate and measure the junction interface. We obtain both the topography and dI/dV spectra at the junction area, from which we track the shift of the graphene chemical potential with position across the junction interface. This allows us to directly measure the spatial width and roughness of the junction and its potential barrier height. We will compare the junction properties of devices fabricated by the aforementioned two methods and discuss their effects on the performance as a Veselago lens.

Nano letters, Jan 11, 2016

The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tun... more The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS2 by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS2. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS2 film at a density of 1.3 × 10(12) eV(-1) cm(-2). By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have str...

Bulletin of the American Physical Society, Mar 5, 2015

Bulletin of the American Physical Society, 2016

Submitted for the MAR16 Meeting of The American Physical Society Raman characterization of few-la... more Submitted for the MAR16 Meeting of The American Physical Society Raman characterization of few-layered 1T'-MoTe 2

Bulletin of the American Physical Society, 2016

APS March Meeting Abstracts, 2016

Bulletin of the American Physical Society, Mar 15, 2016

Bulletin of the American Physical Society, Mar 18, 2016

Bulletin of the American Physical Society, Mar 17, 2016

Bulletin of the American Physical Society, Mar 15, 2016

Nano Letters, Apr 27, 2016

Bulletin of the American Physical Society, Mar 5, 2015

Bulletin of the American Physical Society, Mar 18, 2016

arXiv (Cornell University), Mar 16, 2023

Nano Letters, May 23, 2017

High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such a... more High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such as MoS 2 is key to unlocking their unique electronic and optoelectronic properties for fundamental research and device applications. Despite extensive experimental and theoretical efforts reliable ohmic contact to doped TMDCs remains elusive and would benefit from a better understanding of the underlying physics of the metal-TMDC interface. Here we present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer MoS 2 using ultra-high vacuum scanning tunneling microscopy (UHV-STM). Our measurements reveal that the electronic properties of these junctions are dominated by 2D metal induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (L-DOS) of the MoS 2 within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of ~0.55 nm near mid gap to as long as 2 nm near the band edges and are nearly identical for Au, Pd and graphite contacts, indicating that it is a universal property of the monolayer semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact.

Bulletin of the American Physical Society, 2017

high-quality electrical contact between deposited metals and monolayer transition metal dichalcog... more high-quality electrical contact between deposited metals and monolayer transition metal dichalcogenides remains a challenging problem. In this work, we probe the factors influencing the contact resistance by using atomicresolution scanning probe microscopy and spectroscopy at the interface between a contact metal and highly doped monolayer molybdenum disulfide (MoS 2). Using atomically resolved spectroscopy measurements, band structure evolution is mapped across the edge of the contact and into the MoS 2 sheet. The spectroscopy shows the presence of metal induced gap states (MIGS) in the semiconductor with a decay length of a few angstroms from the interface, which is the first experimental observation of MIGS in a two-dimensional semiconductor. We show that in the highly doped limit, the MIGS dominate the electronic properties and hence the contact resistance, as opposed to band bending. Finally, we describe the difference in MIGS between graphite, gold and palladium electrodes on MoS 2 .

Bulletin of the American Physical Society, 2016

Bulletin of the American Physical Society, 2016

Graphene p-n junctions are essential devices for studying relativistic Klein tunneling and the Ve... more Graphene p-n junctions are essential devices for studying relativistic Klein tunneling and the Veselago lensing effect in graphene. We have successfully fabricated graphene p-n junctions using both lithographically pre-patterned substrates and the stacking of vertical heterostructures. We then use our 4-probe STM system to characterize the junctions. The ability to carry out scanning electron microscopy (SEM) in our STM instrument is essential for us to locate and measure the junction interface. We obtain both the topography and dI/dV spectra at the junction area, from which we track the shift of the graphene chemical potential with position across the junction interface. This allows us to directly measure the spatial width and roughness of the junction and its potential barrier height. We will compare the junction properties of devices fabricated by the aforementioned two methods and discuss their effects on the performance as a Veselago lens.

Nano letters, Jan 11, 2016

The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tun... more The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS2 by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS2. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS2 film at a density of 1.3 × 10(12) eV(-1) cm(-2). By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have str...

Bulletin of the American Physical Society, Mar 5, 2015

Bulletin of the American Physical Society, 2016

Submitted for the MAR16 Meeting of The American Physical Society Raman characterization of few-la... more Submitted for the MAR16 Meeting of The American Physical Society Raman characterization of few-layered 1T'-MoTe 2

Bulletin of the American Physical Society, 2016