Optimum Contact Configurations for Quasi-One-Dimensional Phosphorene Nanodevices.

We employ atomistic quantum transport simulations based on non-equilibrium Green's function (NEGF) formalism of quasi-one-dimensional (quasi-1D) phosphorene, or phosphorene nanoribbons (PNRs), to explore routes towards minimizing contact resistance (RC) in devices based on such nanostructures. The impact of PNR width scaling from ~5.5 nm down to ~0.5 nm, different hybrid edge-and-top metal contact configurations, and various metal-channel interaction strengths on the transfer length and RC is studied in detail. We demonstrate that optimum metals and top-contact lengths exist and depend on PNR width, which is a consequence of resonant transport and broadening effects. We find that moderately interacting metals and nearly edge contacts are optimum only for wider PNRs and phosphorene, providing a minimum RC of ~280 Ωµm. Surprisingly, ultra-narrow PNRs benefit from weakly interacting metals combined with long top contacts that lead to an added RC of only ~2 Ωµm in the 0.49 nm wide quasi-1D phosphorene nanodevice.

Naphthalimide-Piperazine Derivatives as Multifunctional "On" and "Off" Fluorescent Switches for pH, Hg2+ and Cu2+ Ions.

Novel 1,8-naphthalimide-based fluorescent probes NI-1 and NI-2 were designed and screened for use as chemosensors for detection of heavy metal ions. Two moieties, methylpyridine (NI-1) and hydroxyphenyl (NI-2), were attached via piperazine at the C-4 position of the napthalimide core resulting in a notable effect on their spectroscopic properties. NI-1 and NI-2 are pH sensitive and show an increase in fluorescence intensity at around 525 nm (switch "on") in the acidic environment, with pKa values at 4.98 and 2.91, respectively. Amongst heavy metal ions only Cu2+ and Hg2+ had a significant effect on the spectroscopic properties. The fluorescence of NI-1 is quenched in the presence of either Cu2+ or Hg2+ which is attributed to the formation of 1:1 metal-ligand complexes with binding constants of 3.6 × 105 and 3.9 × 104, respectively. The NI-1 chemosensor can be used for the quantification of Cu2+ ions in sub-micromolar quantities, with a linear range from 250 nM to 4.0 µM and a detection limit of 1.5 × 10-8 M. The linear range for the determination of Hg2+ is from 2 µM to 10 µM, with a detection limit of 8.8 × 10-8 M. Conversely, NI-2 behaves like a typical photoinduced electron transfer (PET) sensor for Hg2+ ions. Here, the formation of a complex with Hg2+ (binding constant 8.3 × 103) turns the green fluorescence of NI-2 into the "on" state. NI-2 showed remarkable selectivity towards Hg2+ ions, allowing for determination of Hg2+ concentration over a linear range of 1.3 µM to 25 µM and a limit of detection of 4.1 × 10-7 M.

Lower Limits of Contact Resistance in Phosphorene Nanodevices with Edge Contacts.

Edge contacts are promising for improving carrier injection and contact resistance in devices based on two-dimensional (2D) materials, among which monolayer black phosphorus (BP), or phosphorene, is especially attractive for device applications. Cutting BP into phosphorene nanoribbons (PNRs) widens the design space for BP devices and enables high-density device integration. However, little is known about contact resistance (RC) in PNRs with edge contacts, although RC is the main performance limiter for 2D material devices. Atomistic quantum transport simulations are employed to explore the impact of attaching metal edge contacts (MECs) on the electronic and transport properties and contact resistance of PNRs. We demonstrate that PNR length downscaling increases RC to 192 Ω µm in 5.2 nm-long PNRs due to strong metallization effects, while width downscaling decreases the RC to 19 Ω µm in 0.5 nm-wide PNRs. These findings illustrate the limitations on PNR downscaling and reveal opportunities in the minimization of RC by device sizing. Moreover, we prove the existence of optimum metals for edge contacts in terms of minimum metallization effects that further decrease RC by ~30%, resulting in lower intrinsic quantum limits to RC of ~90 Ω µm in phosphorene and ~14 Ω µm in ultra-narrow PNRs.

Correction: Poljak, M.; Matic, M. Metallization-Induced Quantum Limits of Contact Resistance in Graphene Nanoribbons with One-Dimensional Contacts. Materials 2021, 14, 3670.

The authors regret that the results presented in Figure 3c,d and Figure 6c,d in our published paper [...].

Metallization-Induced Quantum Limits of Contact Resistance in Graphene Nanoribbons with One-Dimensional Contacts.

Graphene has attracted a lot of interest as a potential replacement for silicon in future integrated circuits due to its remarkable electronic and transport properties. In order to meet technology requirements for an acceptable bandgap, graphene needs to be patterned into graphene nanoribbons (GNRs), while one-dimensional (1D) edge metal contacts (MCs) are needed to allow for the encapsulation and preservation of the transport properties. While the properties of GNRs with ideal contacts have been studied extensively, little is known about the electronic and transport properties of GNRs with 1D edge MCs, including contact resistance (RC), which is one of the key device parameters. In this work, we employ atomistic quantum transport simulations of GNRs with MCs modeled with the wide-band limit (WBL) approach to explore their metallization effects and contact resistance. By studying density of states (DOS), transmission and conductance, we find that metallization decreases transmission and conductance, and either enlarges or diminishes the transport gap depending on GNR dimensions. We calculate the intrinsic quantum limit of width-normalized RC and find that the limit depends on GNR dimensions, decreasing with width downscaling to ~21 Ω∙µm in 0.4 nm-wide GNRs, and increasing with length downscaling up to ~196 Ω∙µm in 5 nm-long GNRs. We demonstrate that 1D edge contacts and size engineering can be used to tune the RC in GNRs to values lower than those of graphene.

Bandstructure and Size-Scaling Effects in the Performance of Monolayer Black Phosphorus Nanodevices.

Nanodevices based on monolayer black phosphorus or phosphorene are promising for future electron devices in high density integrated circuits. We investigate bandstructure and size-scaling effects in the electronic and transport properties of phosphorene nanoribbons (PNRs) and the performance of ultra-scaled PNR field-effect transistors (FETs) using advanced theoretical and computational approaches. Material and device properties are obtained by non-equilibrium Green's function (NEGF) formalism combined with a novel tight-binding (TB) model fitted on ab initio density-functional theory (DFT) calculations. We report significant changes in the dispersion, number, and configuration of electronic subbands, density of states, and transmission of PNRs with nanoribbon width (W) downscaling. In addition, the performance of PNR FETs with 15 nm-long channels are self-consistently assessed by exploring the behavior of charge density, quantum capacitance, and average charge velocity in the channel. The dominant consequence of W downscaling is the decrease of charge velocity, which in turn deteriorates the ON-state current in PNR FETs with narrower nanoribbon channels. Nevertheless, we find optimum nanodevices with W > 1.4 nm that meet the requirements set by the semiconductor industry for the "3 nm" technology generation, which illustrates the importance of properly accounting bandstructure effects that occur in sub-5 nm-wide PNRs.