If young children approached the task of word learning with a specific hypothesis about the meaning of novel count nouns, they could make the problem of word learning more tractable. Six experiments were conducted to test children's hypotheses about how labels map to object categories. Findings indicated that (a) 3-and 4-year-olds function with an antithematic bias; (b) children do not reliably extend novel nouns to superordinate exemplars when perceptual similarity is controlled until approximately age 7; and (c) children expect novel nouns to label taxonomic categories at the basic level, even in the presence of a perceptually compelling distractor. Results are interpreted as supporting the principle of categorical scope (R. M. Golinkoff, C. B. Mervis, & K. Hirsh-Pasek, 1994).How do young children, introduced to a new word, avoid the myriad, logically possible assumptions (see Quine, 1960) they might make about what the new word means? That children quickly settle on the meaning of novel words is shown by their rapid progress in vocabulary acquisition: By the end of the second year of life, children at the 50th percentile on the MacArthur Inventory have amassed a productive vocabulary of 324 words (Fenson et al., 1991). To account for the brisk pace of word learning, researchers have suggested that children approach the task with a set of hypotheses about what words mean-hypotheses that bring certain possibilities over others to the top of the stack (e.g., Clark, 1983;Markman, 1989;Merriman & Bowman, 1989; see Golinkoff, Mervis, & Hirsh-Pasek, 1994, for a review). The purpose of the present experiments was to advance prior research (especially that of Markman & Hutchinson, 1984) that focused on children's word learning hypotheses. The first question was whether children even form hypotheses in the presence of a new word or whether there are more parsimonious explanations for their responses. The sec-

Single-layer WS2 has shown excellent photoresponse properties, but its promising applications in high-sensitivity photodetection suffer from the atomic-thickness-limited adsorption and band-gap-limited spectral selectivity. Here we have carried out investigations on WS2 monolayer based phototransistors with and without decoration of SnSe nanocrystals (NCs) for comparison. Compared to the solely WS2 monolayer, SnSe NCs decoration leads to not only huge enhancement of photoresponse in visible spectrum but also extension to near-infrared. Under excitation of visible light in a vacuum, the responsivity at zero gate bias can be enhanced by more than 45 times to ∼99 mA/W, and the response time is retained in millisecond level. Particularly, with extension of photoresponse to near-infrared (1064 nm), a responsivity of 6.6 mA/W can be still achieved. The excellent photoresponse from visible to near-infrared is considered to benefit from synergism of p-type SnSe NCs and n-type WS2 monolayer, or in other words, the formed p-n heterojunctions between p-type SnSe NCs and n-type WS2 monolayer.

Due to the weak screening effect, the concentration and type of charge carriers in 2D semiconductor heterostructures can be effectively tuned by electrostatic gating, enabling us to realize different types of heterojunctions in a single device. Such 'type tunable' properties are useful for designing novel electrical or optoelectrical devices. Here, we demonstrate a 'type tunable' heterojunction device construct with two pieces of ambipolar 2D semiconductors: WSe2 and black phosphorus (BP). This heterojunction could be tuned to either the p-p junction or n-n junction by gate modulation. The p-p junction shows a large current rectification ratio while the n-n junction shows a negligible current rectification ratio, indicating a large valence band offset and a small conduction band offset at the WSe2/BP interface. In the optoelectrical measurements, we found the amplitude and even the polarity of photocurrent could be modulated by electrostatic gating. Our study could further enhance the understanding of designing devices based on these 'type tunable' van der Waals heterojunctions. Moreover, the properties of the WSe2/BP interface were also experimentally identified through the electrical and optoelectrical measurements in our study.

Two-dimensional chalcogenide monolayers are strong candidates for next-generation flexible and transparent optoelectronics. Due to the intrinsic ultrathin thickness and limited optical absorption, however, their responsivity is normally low. Here we develop a simple and low-cost method to fabricate high-performance hybrid phototransistors of monolayer WS with significantly enhanced responsivity and an extended spectral response range, by virtue of surface decoration with liquid-phase exfoliated SnS nanosheets (NSs). The hybrid phototransistors show a much enhanced responsivity of ∼2 A W and an ultrahigh light/dark signal-to-noise ratio of 10 under 457 nm excitation, exhibiting a significant increase of 3 orders of magnitude in responsivity and a 100 fold increase in signal-to-noise ratio, compared with pure WS devices. Our hybrid photodetectors also exhibit a respectable response speed, with a rise and decay time of 51 μs and 98 μs, respectively. After optimal surface decoration with narrow bandgap SnS NSs atop a monolayer WS channel, an emergent optical responsivity in the near infrared region (1064 nm) is also observed.

In this study, high quality GeP5 crystals with two-dimensional (2D) layered structures and novel electrical conductivity of 2.4 × 106 S m−1 have been prepared under high-temperature high-pressure oriented growth technique (HTHP-OGT).