25 research outputs found
Pnictogens Allotropy and Phase Transformation during van der Waals Growth
Pnictogens have multiple allotropic forms resulting from their ns2 np3
valence electronic configuration, making them the only elemental materials to
crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout
the group. Light group VA elements are found in the layered orthorhombic A17
phase such as black phosphorus, and can transition to the layered rhombohedral
A7 phase at high pressure. On the other hand, bulk heavier elements are only
stable in the A7 phase. Herein, we demonstrate that these two phases not only
co-exist during the vdW growth of antimony on weakly interacting surfaces, but
also undertake a spontaneous transformation from the A17 phase to the
thermodynamically stable A7 phase. This metastability of the A17 phase is
revealed by real-time studies unraveling its thickness-driven transition to the
A7 phase and the concomitant evolution of its electronic properties. At a
critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle
transition from AB to AA stacked alpha-antimonene followed by a gradual
relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of
this intermediate phase is found to be determined by surface self-passivation
and the associated competition between A7- and A17-like bonding in the bulk.
These results highlight the critical role of the atomic structure and
interfacial interactions in shaping the stability and electronic
characteristics of vdW layered materials, thus enabling a new degree of freedom
to engineer their properties using scalable processes
Direct oriented growth of armchair graphene nanoribbons on germanium
Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribbons narrower than 10nm with controlled crystallographic orientation and well-defined armchair edges. However, the scalable synthesis of nanoribbons with this precision directly on insulating or semiconducting substrates has not been possible. Here we demonstrate the synthesis of graphene nanoribbons on Ge(001) via chemical vapour deposition. The nanoribbons are self-aligning 3 degrees from the Ge < 110 > directions, are self-defining with predominantly smooth armchair edges, and have tunable width to <10 nm and aspect ratio to >70. In order to realize highly anisotropic ribbons, it is critical to operate in a regime in which the growth rate in the width direction is especially slow, <5 nm h(-1). This directional and anisotropic growth enables nanoribbon fabrication directly on conventional semiconductor wafer platforms and, therefore, promises to allow the integration of nanoribbons into future hybrid integrated circuits
On systems and control approaches to therapeutic gain
BACKGROUND: Mathematical models of cancer relevant processes are being developed at an increasing rate. Conceptual frameworks are needed to support new treatment designs based on such models. METHODS: A modern control perspective is used to formulate two therapeutic gain strategies. RESULTS: Two conceptually distinct therapeutic gain strategies are provided. The first is direct in that its goal is to kill cancer cells more so than normal cells, the second is indirect in that its goal is to achieve implicit therapeutic gains by transferring states of cancer cells of non-curable cases to a target state defined by the cancer cells of curable cases. The direct strategy requires models that connect anti-cancer agents to an endpoint that is modulated by the cause of the cancer and that correlates with cell death. It is an abstraction of a strategy for treating mismatch repair (MMR) deficient cancers with iodinated uridine (IUdR); IU-DNA correlates with radiation induced cell killing and MMR modulates the relationship between IUdR and IU-DNA because loss of MMR decreases the removal of IU from the DNA. The second strategy is indirect. It assumes that non-curable patient outcomes will improve if the states of their malignant cells are first transferred toward a state that is similar to that of a curable patient. This strategy is difficult to employ because it requires a model that relates drugs to determinants of differences in patient survival times. It is an abstraction of a strategy for treating BCR-ABL pro-B cell childhood leukemia patients using curable cases as the guides. CONCLUSION: Cancer therapeutic gain problem formulations define the purpose, and thus the scope, of cancer process modeling. Their abstractions facilitate considerations of alternative treatment strategies and support syntheses of learning experiences across different cancers
Graphene Growth Dynamics on Epitaxial Copper Thin Films
Graphene chemical vapor deposition
on copper is phenomenologically
complex, yielding diverse crystal morphologies, including lobes, dendrites,
stars, and hexagons, of various orientations depending on conditions.
We present a comprehensive study of the evolution of these morphologies
as a function of the copper surface orientation, absolute pressure,
hydrogen-to-methane ratio (H<sub>2</sub>:CH<sub>4</sub>), and nucleation
density. Growth was studied on ultrasmooth, epitaxial copper films
inside copper enclosures to minimize copper polycrystallinity and
roughness and decrease the graphene nucleation density. At low pressure
and low H<sub>2</sub>:CH<sub>4</sub>, circular graphene islands initially
form. After exceeding ∼1.0 μm, Mullins-Sekerka instabilities
evolve into dendrites extending hundreds of micrometers in the ⟨100⟩,
⟨111⟩, and ⟨110⟩ directions on Cu(100),
Cu(110), and Cu(111), respectively, indicating mass transport limited
growth. Twin boundaries perturb the preferential growth direction
on Cu(111) and alter graphene morphology. Increasing H<sub>2</sub>:CH<sub>4</sub> results in compact islands that reflect the copper
symmetry. At atmospheric pressure and low H<sub>2</sub>:CH<sub>4</sub>, Mullins-Sekerka instabilities develop but with multiple preferred
orientations. Increasing H<sub>2</sub>:CH<sub>4</sub> results in more
hexagonal islands. Every growth regime can be tuned to yield continuous
monolayers with a D:G Raman ratio <0.1. The understanding gained
from this study provides a roadmap to rationally tailor the structure,
morphology, and orientation of graphene crystals
High-Performance Charge Transport in Semiconducting Armchair Graphene Nanoribbons Grown Directly on Germanium
The
growth of graphene on Ge(001) <i>via</i> chemical
vapor deposition can be highly anisotropic, affording the facile synthesis
of crystallographically controlled, narrow, long, oriented <i>nanoribbons</i> of graphene that are semiconducting, whereas
unpatterned continuous graphene is semimetallic. This bottom-up growth
overcomes long-standing challenges that have limited top-down ribbon
fabrication (<i>e</i>.<i>g</i>., inadequate resolution
and disordered edges) and yields ribbons with long segments of smooth
armchair edges. The charge transport characteristics of sub-10 nm
ribbons synthesized by this technique (which are expected to have
band gaps sufficiently large for semiconductor electronics applications)
have not yet been characterized. Here, we show that sub-10 nm nanoribbons
grown on Ge(001) can simultaneously achieve a high on/off conductance
ratio of 2 × 10<sup>4</sup> and a high on-state conductance of
5 μS in field-effect transistors, favorably comparing to or
exceeding the performance of nanoribbons fabricated by other methods.
These promising results demonstrate that the direct synthesis of nanoribbons
on Ge(001) could provide a scalable pathway toward the practical realization
of high-performance semiconducting graphene electronics, provided
that the width uniformity and positioning of the nanoribbons are improved
Seed-Initiated Anisotropic Growth of Unidirectional Armchair Graphene Nanoribbon Arrays on Germanium
It
was recently discovered that the chemical vapor deposition (CVD)
of CH<sub>4</sub> on Ge(001) can directly yield long, narrow, semiconducting
nanoribbons of graphene with smooth armchair edges. These nanoribbons
have exceptional charge transport properties compared with nanoribbons
grown by other methods. However, the nanoribbons nucleate at random
locations and at random times, problematically giving rise to width
and bandgap polydispersity, and the mechanisms that drive the anisotropic
crystal growth that produces the nanoribbons are not understood. Here,
we study and engineer the seed-initiated growth of graphene nanoribbons
on Ge(001). The use of seeds decouples nucleation and growth, controls
where growth occurs, and allows graphene to grow with lattice orientations
that do not spontaneously form without seeds. We discover that when
the armchair direction (i.e., parallel to Cî—¸C bonds) of the
seeds is aligned with the Ge⟨110⟩ family of directions,
the growth anisotropy is maximized, resulting in the formation of
nanoribbons with high-aspect ratios. In contrast, increasing misorientation
from Ge⟨110⟩ yields decreasingly anisotropic crystals.
Measured growth rate data are used to generate a construction analogous
to a kinetic Wulff plot that quantitatively predicts the shape of
graphene crystals on Ge(001). This knowledge is employed to fabricate
regularly spaced, unidirectional arrays of nanoribbons and to significantly
improve their uniformity. These results show that seed-initiated graphene
synthesis on Ge(001) will be a viable route for creating wafer-scale
arrays of narrow, semiconducting, armchair nanoribbons with rationally
controlled placement and alignment for a wide range of semiconductor
electronics technologies, provided that dense arrays of sub-10 nm
seeds can be uniformly fabricated in the future
Mechanism of Ultrafast Triplet Exciton Formation in Single Cocrystals of π‑Stacked Electron Donors and Acceptors
Ultrafast
triplet formation in donor–acceptor (D–A)
systems typically occurs by spin–orbit charge-transfer intersystem
crossing (SOCT-ISC), which requires a significant orbital angular
momentum change and is thus usually observed when the adjacent π
systems of D and A are orthogonal; however, the results presented
here show that subnanosecond triplet formation occurs in a series
of D–A cocrystals that form one-dimensional cofacial π
stacks. Using ultrafast transient absorption microscopy, photoexcitation
of D–A single cocrystals, where D is coronene (Cor) or pyrene
(Pyr) and A is N,N-bis(3′-pentyl)-perylene-3,4:9,10-bis(dicarboximide)
(C5PDI) or naphthalene-1,4:5,8-tetracarboxydianhydride
(NDA), results in formation of the charge transfer (CT) excitons Cor•+-C5PDI•–, Pyr•+-C5PDI•–, Cor•+-NDA•–, and Pyr•+-NDA•– in <300 fs, while triplet exciton
formation occurs in Ï„ = 125, 106, 484, and 958 ps, respectively.
TDDFT calculations show that the SOCT-ISC rates correlate with charge
delocalization in the CT exciton state. In addition, time-resolved
EPR spectroscopy shows that Cor•+-C5PDI•– and Pyr•+-C5PDI•– recombine to form localized 3*C5PDI excitons with zero-field splittings of |D| = 1170 and 1250 MHz, respectively. In contrast, Cor•+-NDA•– and Pyr•+-NDA•– give triplet excitons in which |D| is only 1240 and 690 MHz, respectively, compared to that of NDA
(2091 MHz), which is the lowest energy localized triplet exciton,
indicating that the Cor-NDA and Pyr-NDA triplet excitons have significant
CT character. These results show that charge delocalization in CT
excitons impacts both ultrafast triplet formation as well as the CT
character of the resultant triplet states